Preheat your oven to 375°F (190°C) and lightly grease a baking dish.
Slice the bell peppers in half lengthwise, removing the seeds and membranes, then place them cut-side up in the prepared dish.
In a large skillet, heat the olive oil over medium heat and sauté the diced yellow onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it apart with a spoon, and cook until fully browned and no longer pink.
Stir in the cooked brown rice, tomato sauce, dried oregano, sea salt, and black pepper, simmering for 2 minutes to meld the flavors.
Spoon the beef and rice mixture generously into each bell pepper cavity.
Sprinkle the grated parmesan cheese evenly over the tops of the stuffed peppers.
Cover the baking dish with foil and bake for 20 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are tender and the cheese is golden.
Let the peppers rest for 5 minutes before serving to allow the juices to settle.