Preheat the oven to 375°, convection bake. Preheat a cast iron skillet or large oven safe fry pan over medium to medium high heat. Rub the pork on all sides with salt, pepper, and brown sugar. Add oil to the hot skillet and sear the pork until golden brown on all sides, including the sides and ends (hold it up with tongs), about 6 minutes. The brown sugar will want to burn, flip it just before it does!
Add the raw sliced onions to the skillet around the pork and season them with a pinch of salt. Transfer the skillet to the oven and roast for about 40-50 minutes. Take it out half way through, flip the pork and give the onions a toss.
After 30 to 40 minutes, use a thermometer to check the internal temperature has reached 145℉. If it hasn't, continue to cook and recheck every 5 minutes. The roasting time can vary depending on the size of the roast, the oven, and how long the meat was seared, so begin checking after 30 minutes. If the onions are starting to burn before the meat is done, you can remove them to a bowl with tongs. If the onions aren't done, remove the meat to a cutting board and let them keep cooking.
Meanwhile combine all ingredients for the garlic honey butter and mix well with a fork. Slice open your hoagie rolls and slather on some of the butter. Let them toast in the oven a few minutes as the pork is almost done. (If you love the flavored butter, consider doubling the amounts!)
When the internal temperature reaches 145°, remove the pork from the oven and let it rest in the skillet for 10 minutes before slicing. You can add the rest of the honey garlic butter to melt down over the pork roast.
With a sharp knife or serrated bread knife, slice the pork as thin as possible, shaving it thin so it is falling apart a bit. Pile on the toasted hoagies and top with the crispy caramelized onions.