Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and internal membranes.
Finely dice the yellow onion and mince the garlic clove.
In a large skillet, heat the olive oil over medium heat and sauté the onion and garlic until they become translucent and aromatic.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until it is fully browned.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper, then simmer the mixture for 3 minutes to meld the flavors.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to ensure they are well-filled.
Place the stuffed peppers upright in a baking dish, add 2 tablespoons of water to the bottom of the dish, and cover tightly with foil.
Bake for 30 minutes, then remove the foil and continue baking for an additional 10 minutes until the peppers are tender and the tops are slightly golden.