Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers, remove the seeds and membranes, and set aside.
In a large skillet, heat the olive oil over medium heat. Add the diced yellow onion and sauté for 3-4 minutes until softened.
Add the minced garlic and cook for another 30 seconds until fragrant.
Add the lean ground beef to the skillet, breaking it up with a spatula. Cook until the beef is fully browned and no longer pink.
Stir in the cooked brown rice, tomato sauce, dried oregano, sea salt, and black pepper. Simmer the mixture for 3 minutes to allow the flavors to meld.
Place the bell peppers upright in a baking dish. Spoon the beef and rice mixture into each pepper, pressing down gently to fill them completely.
Pour 1/4 cup of water into the bottom of the baking dish, cover tightly with foil, and bake for 30-35 minutes or until the peppers are tender.
Remove from the oven and garnish with freshly chopped parsley before serving.