Preheat the oven to 350°F.
In a large skillet, heat the olive oil over medium heat. Add the sliced onion and bell pepper, cooking until softened, about 5-7 minutes.
In a large bowl, combine the cooked chicken, black beans, diced tomatoes, chili powder, cumin, garlic powder, salt, and black pepper. Stir in the cooked rice and the sautéed vegetables.
Transfer the mixture to a greased 9x13-inch baking dish. Spread it evenly.
Sprinkle the shredded cheddar cheese on top of the casserole.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let it cool for a few minutes. Serve with sour cream and garnish with fresh cilantro if desired. Enjoy with tortilla chips for added crunch.