Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes. If necessary, cut a thin slice off the bottom so they stand upright.
In a large skillet over medium heat, add the olive oil and sauté the diced yellow onion until translucent, about 4 minutes.
Add the minced garlic and ground beef to the skillet. Cook until the beef is browned, breaking it up with a spoon as it cooks.
Drain any excess fat from the pan, then stir in the cooked brown rice, tomato puree, sea salt, black pepper, and dried oregano.
Simmer the mixture for 2-3 minutes until well combined and slightly thickened.
Stand the bell peppers in a baking dish. Spoon the beef and rice mixture into each pepper until they are tightly packed.
Top each pepper with a sprinkle of grated parmesan cheese.
Pour a small amount of water (about 1/4 cup) into the bottom of the baking dish to help the peppers steam.
Cover the dish with foil and bake for 30 minutes. 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.