Preheat your oven to 375°F and prepare a small baking dish.
Slice the tops off the bell peppers, remove the seeds and membranes, and set the peppers upright in the baking dish.
Heat the olive oil in a skillet over medium heat, then add the diced onion and minced garlic, sautéing until translucent.
Add the ground beef to the skillet, seasoning with sea salt, black pepper, and dried oregano, and cook until fully browned.
Stir the cooked brown rice and half of the tomato puree into the beef mixture until well incorporated.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to fill them completely.
Pour the remaining tomato puree into the bottom of the baking dish around the peppers.
Top each pepper with parmesan cheese, cover the dish tightly with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are soft and the cheese is lightly golden.