Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and membranes from the inside.
Finely dice the yellow onion and mince the garlic clove.
Heat the olive oil in a large skillet over medium heat, then add the onion and garlic, sautéing until translucent.
Add the ground beef to the skillet and cook until fully browned, breaking it up with a spoon as it cooks.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper until well combined.
Stand the peppers upright in a small baking dish and fill each one generously with the beef and rice mixture.
Pour a small amount of water (about 2 tablespoons) into the bottom of the baking dish to help steam the peppers.
Cover the dish with foil and bake for 30 minutes, then remove the foil, sprinkle with parmesan cheese, and bake for another 5 minutes until the peppers are soft and the cheese is melted.