Preheat oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes.
In a large skillet over medium heat, add the olive oil and sauté the diced onion and minced garlic until fragrant and translucent.
Add the ground beef to the skillet, breaking it up with a 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 until well combined.
Place the hollowed bell peppers in a baking dish and spoon the beef and rice mixture into each pepper until tightly packed.
Sprinkle the grated parmesan cheese over the top of each stuffed pepper.
Add a small splash of water to the bottom of the baking dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the cheese is golden.