In a large skillet, heat the olive oil over medium heat. Add the sliced bell peppers and onion. Sauté for about 5-7 minutes until softened.
In a large mixing bowl, combine the cooked chicken, sautéed vegetables, cooked brown rice, black beans, corn, salsa, cumin, chili powder, garlic powder, and onion powder. Mix until well combined.
Transfer the mixture into a greased 9x13 inch baking dish. Spread it evenly.
Spread the sour cream over the top of the casserole, then sprinkle the shredded cheddar cheese evenly on top.
Bake in the preheated oven for about 25-30 minutes, or until the cheese is bubbly and slightly golden.
Remove from the oven and let it cool for about 5 minutes before serving.
Garnish with fresh cilantro before serving for an extra burst of flavor.