Preheat and Prep: Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Place the halved bell peppers cut-side up in the dish (or whole).
Sauté Aromatics: Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 2-3 minutes until soft and translucent. Add the minced garlic and cook for 1 more minute until fragrant.
Cook the Chicken: Add the ground chicken, dried basil, dried oregano, salt, and black pepper to the skillet. Cook, breaking the meat apart with a wooden spoon, until the chicken is browned and no longer pink (about 6-8 minutes).
Mix the Filling: Remove the skillet from the heat.
Stir in the cooked brown rice (or quinoa) and 1/2 cup of the marinara sauce.
Stuff the Peppers: Spoon the chicken and rice mixture evenly into each pepper half, pressing down gently to fill them. Spoon a little extra marinara sauce over the top of each stuffed pepper.
Bake: Cover the dish loosely with foil and bake for: 25 minutes. Remove the foil, top each pepper generously with the shredded mozzarella cheese, and bake uncovered for another 5-10 minutes until the peppers are tender and the cheese is melted & golden.
Serve: Garnish with fresh chopped basil or parsley.