Preheat the oven to 375°F (190°C). Place the halved, seeded bell peppers cut side up in a baking dish.
Heat the olive oil in a large skillet over medium heat.
Add the chopped onion and sauté about 3 minutes until softened, then stir in the minced garlic.
Add the diced chicken to the skillet and season with chili powder, cumin, salt, and pepper; cook 5–7 minutes until the chicken is browned and cooked through.
Stir in the cooked brown rice and drained diced tomatoes until evenly combined and heated through.
Spoon the chicken and rice mixture evenly into the pepper halves.
Cover the baking dish with foil and bake for 20 minutes.
Remove the foil, sprinkle the shredded cheddar over each pepper, and bake uncovered 5–10 minutes more until the cheese is melted and peppers are tender.
Garnish with fresh cilantro if desired and serve warm.