Preheat your oven to 375°F (190°C) and line a baking dish with parchment paper.
Slice the bell peppers in half lengthwise, removing the seeds and membranes while keeping the stems intact for structure.
In a large skillet over medium heat, add the olive oil and sauté the diced onion and minced garlic until they become translucent and fragrant.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until browned and no longer pink.
Stir in the cooked brown rice, tomato puree, sea salt, black pepper, and dried oregano, allowing the flavors to meld for 2 minutes.
Spoon the beef and rice mixture generously into the pepper halves and place them in the prepared baking dish.
Sprinkle the grated parmesan cheese over the top of each stuffed pepper.
Bake for 25 to 30 minutes until the peppers are tender and the cheese has transformed into a beautiful golden crust.