Preheat your oven to 375 degrees Fahrenheit and lightly grease a small baking dish.
Slice the tops off the bell peppers, remove the seeds and membranes, and set the peppers upright in the dish.
In a large skillet over medium heat, add the olive oil and sauté the diced onion and minced garlic until translucent.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until it is completely browned.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper until the mixture is well combined.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to pack them tightly.
Pour two tablespoons of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil, sprinkle each pepper with parmesan cheese, and bake for another 10 minutes until the peppers are tender and the cheese is golden.