Preheat your oven to 375°F (190°C). Slice the tops off the bell peppers and remove the seeds and white ribs 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, about 4-5 minutes.
Add the minced garlic and the lean ground beef to the skillet, breaking the meat apart with a wooden spoon until it is fully browned.
Stir in the cooked brown rice, tomato puree, sea salt, black pepper, and dried oregano, mixing until the ingredients are well incorporated and fragrant.
Stand the bell peppers upright in a small baking dish. If they are wobbly, trim a tiny bit off the bottom without creating a hole.
Spoon the beef and rice mixture into the peppers, packing it down gently to fill them to the top.
Pour 0.25 cup of water into the bottom of the baking dish, cover tightly with aluminum foil, and bake for 30 minutes.
Remove the foil and continue baking for another 10 minutes until the peppers are tender and the tops are slightly golden.