Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes from the inside.
In a large skillet over medium heat, add the olive oil and sauté the diced yellow onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until browned and cooked through.
Stir in the cooked brown rice, tomato sauce, dried oregano, sea salt, and black pepper, allowing the mixture to simmer for 2-3 minutes.
Stand the bell peppers upright in a baking dish and spoon the beef and rice mixture into each pepper until full.
Top each pepper with the grated parmesan cheese.
Add about 0.25 cup of water to the bottom of the baking dish to help the peppers steam.
Cover the dish with foil and bake for 30 minutes, then remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the cheese is golden.
Let the peppers rest for 5 minutes before serving.