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