Green Bean Au Gratin is the perfect vegetable side dish for your holiday table! An all around better version of green bean casserole, with no condensed soup or mushy canned green beans! This elegant dish starts with caramelized onions and garlic in a creamy milk sauce, made easy with frozen green beans, and baked with a cheesy bread crumb crust!
No matter the holiday, this Green Bean Au Gratin pairs well with turkey, ham, steaks or any main course! It can be made ahead and baked on the busy day.
Since green beans from the store are usually pretty terrible in the winter, this recipe uses frozen green beans, which are already blanched and ready to bake. You could also blanch and freeze your summer garden green beans and save them for this holiday dish!
This frozen green bean casserole is rich and creamy made with butter and milk, while the green beans stay perfectly tender and never mushy! You'll want to make this one year after year!
If you're a gardener, we love to Blanch and Freeze Garden Green Beans to use all winter long and to make this Green Bean Au Gratin for the holidays!
Ingredients & Substitutions
Frozen green beans, caramelized onions, and a simple bechamel sauce (a white sauce made from butter, flour, and milk) give this clean ingredient list amazing flavor!
- Onions & Garlic - This recipe starts by caramelizing an onion in melted butter. Caramelizing is the "low and slow" process of sautéing an onion over low heat to develop its natural sugars. It will turn deeply golden brown in color and take on amazing flavor. Garlic is then added after the onion has caramelized.
- Butter & Flour - It may seem like too much butter when caramelizing the onions, but you need it! Equal amounts of butter and flour will form a roux, the thickener for the milk sauce. Be sure to cook the garlic and flour together for a minute to get rid of the raw flour taste.
- Milk - Use whole milk if you have it, or at least 2%. If all you have is skim, it would be great to add a splash of heavy cream. The milk is whisked with the roux, thickening it into a bechamel, a basic white sauce that will be the base of the gratin.
- Seasonings - Salt and pepper are extremely important to this simple dish. A bit of thyme and nutmeg are classic in a creamy bechamel sauce. Nutmeg adds a warm rich undertone, but if you don't have it, it's ok to leave out.
- White Cheddar & Parmesan Cheese - An affordable, melty cheese, white cheddar, along with Parmesan cheese bake into the perfect topping! Gruyere cheese could be substituted, common and delicious in au gratins, but much more expensive.
- Frozen Cut Green Beans - I chose frozen green beans over fresh for this recipe because green beans can be tough to find in the winter, and usually are tough and not very good. Plus, it's just quick and easy! No need to thaw or chop, just toss them in frozen! Frozen green beans are blanched, which means they are pre-cooked, and bake up perfectly al dente, never mushy like canned green beans. I always blanch and freeze my own garden green beans in the summer, measure out and save some for this holiday dish!
- Bread Crumbs - Bread crumbs along with the cheesy topping gives this Green Bean Au Gratin its signature golden brown top! You can easily make your own breadcrumbs by pulsing a chopped bun or any bread from your freezer in the food processor! Use store-bought panko or make your own, but avoid the regular, completely powdery breadcrumbs. They won't give you the right texture.
See recipe card for quantities.
Rustic yet elegant, buttery caramelized onions form the base for a bechamel, a simple white sauce. Toss with the frozen green beans, top with cheese and bread crumbs and bake until golden!
Step 1 - Melt the butter in a skillet over medium low heat and caramelize the onion slices. This is a slow process, bringing out the natural sugars in the onion until golden and delicious! Keep the onions at a gentle sizzle and don't let them burn! Stir often.
Step 2 - Stir often. After about 10 to 15 minutes, they should be a deep golden brown. Pre-measure your salt and pepper and add a good pinch to your cooking onions.
Step 3 - Add the garlic and flour to form a roux with the butter. Stir to combine for about a minute, this will be the thickener for your sauce.
Step 4 - Add the milk and seasonings, including the rest of the salt and pepper.
Step 5 - Whisk often as the sauce thickens, about 5 minutes.
Step 6 - Mix in half the cheese and the frozen green beans, no need to thaw.
Step 7 - Transfer to a baking dish and sprinkle evenly with the remaining cheese. Melt 1 Tablespoon of butter and mix with the bread crumbs and spoon over the top.
Step 8 - Bake at 400° F about 30 minutes until golden and bubbly. You can toast up the top under the broiler for a few minutes but watch it so it doesn't burn!
- Make Ahead Instructions - This dish can be assembled unbaked a day in advance, wrap in foil and refrigerated. Let it sit on the counter a half hour or so before baking to take the chill off. Bake without the foil.
- Travel Instructions - If you are headed to a holiday party, you can bake the gratin at home, cover it with foil and a dish towel and transport it in an insulated bag. Then just keep it warm in the hosts oven.
- The Right Size Baking Dish - You may want to dump your frozen green beans into an empty baking dish to eyeball the amount when deciding what dish to use. I used a 8x8 square dish to serve about 6 people. A larger 9x13 casserole dish, "a lasagna pan" could be used to double the recipe.
- Why Frozen Green Beans? - Fresh green beans are in season in the summer. The fresh grocery store green beans in the winter, when you are likely to make this dish, are usually not very good. I love to blanch and freeze green beans from my garden to eat all winter. You could chop and measure out 4 cups to make this recipe for the holidays! But, store-bought frozen green beans are really a great product. Far better than canned green beans in my opinion, and bake up tender and al dente in this au gratin!
- Don't skimp on the butter! - It feels like more than enough butter when you are caramelizing the onions, but the real purpose of the butter is to form the roux. Together with the flour, this is what thickens your milk sauce.
- Shred your own Cheese - A block of white cheddar cheese is super versatile, I use it for tacos, pizza, omelets, etc. and have it in my fridge all the time! Shredding your own is going to melt up nicer with better texture than the pre-shredded bags.
- Storage - Leftover Green Bean Au Gratin can be store in the fridge for 3 or 4 days.
Serve It With
- Add a starch like my Rosemary Garlic Mashed Potatoes or a Sweet Potato Casserole with Maple Pecan Topping. All are great with any main course including turkey, ham, or beef!
- Try a holiday appetizer like this Cranberry Honey Baked Brie or this Raisin Pine Nut Goat Cheese Appetizer dip too!
- A festive salad like this Pear & Blue Cheese Salad or this Blueberry Goat Cheese Salad are gorgeous for the holidays.
You can, but you will need to blanch or steam them first. Trim the ends and chop fresh green beans into 1 inch pieces. Drop them into boiling salted water for about 3 or 4 minutes. Drain well before adding to the sauce. Be warned that large overgrown green beans are usually tough and not very good out of season. Frozen green beans are recommended when making Green Bean Au Gratin in the winter.
No, you can add the frozen green beans right to your sauce without thawing. Much easier than using fresh green beans that are out of season in the winter anyway!
Caramelized onions are seriously delicious, but if you want to skip the step and make this recipe even easier you can. Just melt the butter and incorporate the flour and then the milk. Add 1 teaspoon each of garlic powder and onion powder if you really want to avoid any chopping. Your dish will have a different flavor but still be good!
Yes, you can freeze the unbaked green bean au gratin, wrapped well in foil and then tied up in a bread bag. Thaw overnight or two nights and bake as instructed. You can also freeze leftover baked Green Bean Au Gratin for up to 3 months. Just thaw overnight in the fridge and reheat.
More Vegetable Side Dish Recipes
If you made this recipe, I'd love to hear how you liked it! Be sure to leave a star rating by clicking the stars in the recipe card, or leave a comment below!
Green Bean Au Gratin
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- 4 Tablespoons butter
- 1 onion sliced
- ½ teaspoon kosher salt
- 8 grinds fresh black pepper
- 3 cloves garlic minced
- 4 Tablespoons flour
- 2 cups milk
- ½ teaspoon dried or fresh thyme
- ⅛ teaspoon nutmeg
- 20 ounces frozen green beans (about 4 to 5 cups)
- 2 cups white cheddar cheese shredded, divided
- ½ cup Parmesan cheese shredded, divided
- 1 Tablespoon butter
- ½ cup bread crumbs
- In a large skillet over medium low heat, melt 4 Tablespoons of butter and caramelize the sliced onions. This is a slow process, about 10 to 15 minutes, stirring often until golden brown. They should sizzle gently, turn the heat down if they begin to burn! Pre-measure your salt and pepper and add a pinch to the onions.
- When the onions are golden and caramelized, add the garlic and flour and cook another minute to incorporate. This will form a roux, the thickener for the sauce.
- Add the milk, thyme, nutmeg and the rest of the salt and pepper. Whisk together over medium low heat until thickened, about 5 minutes.
- When the sauce has thickened, add the frozen green beans (no need to thaw) and half the cheese. Mix well.
- Transfer to a baking dish and smooth the top, it will be very thick. Cover with the rest of the cheese. Melt 1 Tablespoon of butter and toss with the bread crumbs and sprinkle over the top.
- Bake at 400℉ for about 30 minutes until golden and bubbly. You can broil it a few minutes to get the top a golden brown.
- Baking Dish - I used an 8x8 square baking dish. You can dump your frozen green beans into the dish to eyeball the right size to use. You can double the recipe for a larger group.
- Bread Crumbs - Use store-bought panko bread crumbs or make your own by pulsing a chopped bun or any bread from your freezer in the food processor! Avoid the regular, completely powdery breadcrumbs. They won't give you the right texture.
- Milk - Whole milk is best, but 2% is fine. If all you have is skim, you may want to add a splash of cream or sub a bit with half & half.
- Green Beans - I used two 10 ounce bags of cut frozen green beans. You can also use blanched and frozen green beans from your summer garden, just measure out 4 cups.
- If using fresh green beans, blanch them first in boiling salted water for about 3 minutes. Don't use overgrown large fibrous green beans. Their texture is terrible, no matter how long you cook them!