Preheat your oven to 375°F (190°C) and lightly grease a baking dish.
Slice the tops off the bell peppers and carefully 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 until translucent and fragrant.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until fully browned.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for one additional minute.
Remove from heat and fold in the cooked brown rice and tomato puree until the mixture is well combined.
Spoon the beef and rice mixture into the prepared bell peppers, packing it down gently.
Place the peppers upright in the baking dish, add a splash of water to the bottom of the dish, and cover with foil.
Bake for 30 minutes, then remove the foil and bake for another 5-10 minutes until the peppers are tender.