Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers, remove the seeds and membranes, and set the peppers aside.
In a large skillet, heat the olive oil over medium heat and sauté the diced onion and minced garlic until they become translucent and fragrant.
Add the lean ground beef to the skillet, seasoning with sea salt, black pepper, and dried oregano. Cook until the beef is fully browned, breaking it up with a spatula.
Stir the cooked brown rice and tomato puree into the beef mixture, letting it simmer for 2 minutes to combine the flavors.
Stand the bell peppers upright in a baking dish. If they are wobbly, trim a tiny bit off the bottom without cutting through the base.
Spoon the beef and rice mixture into each pepper until they are tightly packed.
Pour a small amount of water into the bottom of the baking dish (about 1/4 inch) to create steam, then cover the dish tightly with foil.
Bake for 30 to 35 minutes, or until the peppers are tender when pierced with a fork.