Preheat your oven to 375°F.
Slice the tops off the bell peppers and carefully remove the seeds and white membranes from the inside.
In a large skillet over medium heat, sauté the diced yellow onion until it becomes soft and translucent.
Add the ground beef and minced garlic to the skillet, breaking the meat apart with a spatula and cooking until fully browned.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper.
Simmer the mixture for about 3 to 5 minutes to allow the flavors to meld together.
Stand the bell peppers upright in a baking dish and fill each pepper generously with the beef and rice mixture.
Sprinkle the tops of the stuffed peppers with the grated parmesan cheese.
Add a small splash of water to the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and continue baking for an additional 10 minutes until the peppers are tender and the cheese is slightly golden.