In a large skillet, heat the olive oil over medium-high heat. Add the chicken pieces and sprinkle with fajita seasoning. Cook until browned and cooked through.
Add the sliced bell peppers and onion to the skillet. Cook until softened, about 5-7 minutes.
Stir in the black beans and diced tomatoes with green chilies. Bring to a simmer.
Remove the skillet from the heat. Stir in 2 cups of the Monterey Jack cheese and 1/2 cup of the cheddar cheese until melted.
Pour the mixture into a 9×13 inch baking dish.
Top with the remaining Monterey Jack cheese and cheddar cheese.
Bake for 20-25 minutes, or until the cheese is melted and bubbly and the casserole is heated through.
Let it rest for a few minutes before serving. Add your favorite toppings.
Notes
For a spicier casserole, add a pinch of cayenne pepper or a diced jalapeño with the vegetables.
Feel free to substitute other vegetables like corn or zucchini.
This casserole can be assembled ahead of time and baked when ready.