Preheat oven to 350˚F.
Add oil to a large skillet and set over medium heat.
Add diced onions and peppers to the heated oil and cook until soft, about 5 minutes. Add the garlic and cook for 1 more minute.
Add the beans, corn, chicken, and chilis. Season with salt, pepper, and taco seasoning and continue to cook for 5 minutes, or until warmed through.
Reserve 1/2 cup of the enchilada sauce and add the remaining enchilada sauce to the skillet and combine.
Spread the reserved enchilada sauce on the bottom of a 9 x 13 baking dish.
Place two tortillas on top of the sauce so that they cover the bottom of the baking dish.
Spread 1/3 of the chicken mixture over the tortillas.
Sprinkle 1/3 of the shredded white cheddar and cheddar cheese.
Repeat the tortilla, chicken, and cheese layers two more times to make 3 layers in total, ending with cheese.
Bake for 30 minutes, or until casserole is bubbly and cheese is melted.
Serve with sour cream, cilantro, and avocado.