Preheat your oven to 375°F (190°C).
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, add the olive oil and sauté the diced yellow onion until it becomes translucent and soft.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until it is thoroughly browned.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, and cook for 1 minute until fragrant.
Mix the cooked brown rice and half of the tomato sauce into the beef mixture until well combined and heated through.
Place the hollowed-out bell peppers upright in a baking dish and fill each one generously with the beef and rice mixture.
Pour the remaining tomato sauce over the tops of the stuffed peppers and sprinkle evenly 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 bake for an additional 10 minutes until the peppers are tender and the cheese on top is golden brown.