Season the chicken with salt and pepper. Preheat a large skillet over medium heat. Add 1 tablespoon of the oil and let heat for a minute or two.
Add chicken to the skillet and cook 6-8 minutes per side, until the chicken is cooked through.
Remove the chicken from the skillet and place in a large bowl. Shred with two forks and set aside.
Add an additional tablespoon of oil to the pan with the onions and peppers. Cook for 5-7 minutes, until the onions are translucent and the peppers are soft.
Add the minced garlic to the pan and cook for 1 minute, stirring frequently to make sure the garlic does not burn.
Return the shredded chicken to the pan. Add corn and black beans and stir to combine. Cook an additional 1-2 minutes.
Stir in 1 1/2 cups of the enchilada sauce, reserving the remaining 1/2 cup for later. Mix to combine. Remove the pan from the heat.
To make the casserole, pour 1/4 cup of the remaining enchilada sauce into an 8x13-inch casserole dish.
Cover the sauce completely with 1/3 of the corn tortillas. Spoon 1/2 of the chicken mixture onto the corn tortillas and sprinkle with 1/3 of the shredded cheese. Repeat again with an additional 1/3 of the corn tortillas, remaining chicken mixture, and 1/3 of the cheese. Top with remaining corn tortillas, remaining 1/4 cup of enchilada sauce, and remaining 1/3 of the cheese.
Bake at 350F for 25-30 minutes, until cheese is completely melted. Remove from the oven and top with sliced avocado and chopped cilantro. Serve immediately.