Preheat your oven to 375°F.
Slice the tops off the bell peppers and carefully remove the seeds and internal ribs.
In a large skillet over medium heat, sauté the diced yellow onion and minced garlic in olive oil until the onion becomes translucent.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until it is fully browned.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper, mixing until the ingredients are well combined and heated through.
Spoon the beef and rice mixture into the hollowed-out bell peppers, pressing down gently to pack them.
Place the stuffed peppers upright in a baking dish and sprinkle the tops with parmesan cheese.
Pour about 0.25 cup of water into the bottom of the baking dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and the cheese is golden.
Garnish with fresh chopped parsley before serving.