Preheat your oven to 375°F (190°C) and lightly grease a small baking dish.
Slice the tops off the bell peppers, remove the seeds and membranes, and set the hollowed peppers aside.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until browned and fully cooked through.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper, simmering for 2-3 minutes to meld the flavors.
Spoon the beef and rice mixture into the prepared bell peppers, packing them tightly.
Place the stuffed peppers upright in the baking dish and sprinkle the tops with the grated parmesan cheese.
Cover the dish with foil and bake for 25 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are tender and the cheese is golden.
Let the peppers rest for 5 minutes before serving warm.