Preheat your oven to 375°F (190°C).
Slice the bell peppers in half lengthwise, removing the seeds and membranes to create a hollow cavity.
Place the pepper halves in a baking dish, cut-side up, and set aside.
In a large skillet over medium heat, add the olive oil and sauté the diced onion and minced garlic until soft and 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 sauce, dried oregano, sea salt, and black pepper until well combined.
Spoon the beef and rice mixture generously into each bell pepper half.
Sprinkle the grated Parmesan cheese evenly over the top of the stuffed peppers.
Cover the baking dish with foil and bake for 20 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and the cheese is slightly golden.
Serve warm, optionally garnished with fresh parsley.