Make the Refrigerator Pickled Red Onions. They take 10 minutes to make and can be made up ahead or time, or just left pickling on the counter while you make the tacos. Preheat the oven to 400℉ and preheat a cast iron skillet on the stovetop over medium heat.
Mix the blackening seasoning in a bowl and sprinkle over the chicken, evenly coating it on both sides. Add the olive oil to the hot skillet and sear the chicken, about 2 to 3 minutes per side (don't move the chicken around while it's searing). Then transfer the skillet to the oven and bake for 10 minutes or until the chicken is cooked through with an internal temperature of 165℉.
Meanwhile, mix all ingredients for the Cotija Corn Salsa in a bowl.
When the chicken is cooked through, allow the skillet to rest on the stovetop for 5 minutes. Slice the chicken on a cutting board.
Tortillas can be warmed in the hot oven or over a gas burner.
Assemble the tacos on tortillas with sliced chicken, Cotija Corn Salsa, avocado slices, and pickled red onions. Garnish with lime wedges and cilantro leaves.