In a large mixing bowl, combine the cooked brown rice, black beans, and corn. Mix well.

In a skillet, heat the olive oil over medium heat. Add cumin, chili powder, garlic powder, salt, and black pepper. Stir for 1-2 minutes until fragrant.

Pour the spice mixture over the rice and bean mixture. Stir to combine evenly.

Divide the rice mixture into bowls. Top with cherry tomatoes, red bell pepper, avocado, red onion, and cilantro.

Serve with sour cream and shredded cheese on the side.