Preheat your oven to 400°F (200°C).

In a 9x13-inch baking dish, combine onion, red bell pepper, brown rice, fajita seasoning, and cayenne pepper.

In a separate bowl, whisk together chicken broth, mild green chilies, tomato paste, and olive oil.

Pour the broth mixture into the rice and vegetable blend, and stir until combined.

Fold in shredded chicken, black beans, and corn kernels.

Cover the baking dish with aluminum foil and bake for 65-70 minutes.

Remove the foil, sprinkle cheese over the casserole, and bake for an additional 5-10 minutes until cheese is melted.

Allow the casserole to cool for 5-10 minutes before serving.

Garnish with cilantro, salsa, and avocado slices if desired.