Preheat your oven to 375°F (190°C) and lightly grease a small baking dish.
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 onion until translucent, about 3-4 minutes.
Add the minced garlic and ground beef to the skillet, breaking the meat apart with a spoon until fully browned.
Stir in the cooked brown rice, tomato puree, sea salt, black pepper, and dried oregano, simmering for 2 minutes to meld flavors.
Pack the beef and rice mixture tightly into the hollowed bell peppers and place them upright in the prepared baking dish.
Add two tablespoons of water to the bottom of the dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and the tops are slightly golden.