Preheat the oven to 375°F and lightly grease a small baking dish.
Slice the tops off the bell peppers, remove the seeds and membranes, and set the peppers aside.
In a large skillet over medium heat, add the olive oil and sauté the diced onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it up with a spatula, and cook until fully browned and no longer pink.
Stir in the cooked brown rice, dried oregano, sea salt, black pepper, and 2 tablespoons of the tomato puree until well combined.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to pack them tightly.
Place the peppers upright in the baking dish, top each with the remaining tomato puree, and bake for 30 to 35 minutes until the peppers are tender.
Garnish with chopped fresh parsley before serving.