Add the chopped onion and sauté for about 3 minutes until it softens. Stir in the minced garlic and diced chicken. Season with chili powder, cumin, salt, and pepper.
Cook until the chicken is browned and cooked through, about 5-7 minutes. Mix in the cooked brown rice and diced tomatoes, stirring to incorporate the flavors.
Spoon the chicken and rice mixture evenly into the pepper halves. Cover the dish with foil and bake for 20 minutes.