Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and inner membranes.
Heat the olive oil in a large skillet over medium heat and sauté the diced yellow onion until it becomes translucent.
Add the lean ground beef to the skillet, breaking it into small crumbles with a wooden spoon, and cook until it is fully browned.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for about 1 minute until fragrant.
Add the cooked brown rice and tomato sauce to the beef mixture, stirring well to combine and heating through for 2 minutes.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to pack them tightly.
Place the stuffed peppers upright in a baking dish and add 2 tablespoons of water to the bottom of the dish.
Cover the dish tightly with foil and bake for 30 minutes.
Remove the foil, sprinkle the tops with grated parmesan cheese, and bake for an additional 10 minutes until the peppers are tender and the cheese is golden.