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 internal seeds and membranes.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion for 4 minutes until translucent.
Add the ground beef to the skillet, breaking it up with a spatula, and cook until browned and fully cooked through.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for 1 minute until highly aromatic.
Remove the skillet from the heat and stir in the cooked brown rice and tomato puree until the mixture is well combined.
Spoon the beef and rice filling into the hollowed bell peppers, pressing down firmly to pack the mixture.
Place the peppers upright in the baking dish, add two tablespoons of water to the bottom of the dish, and cover tightly with foil.
Bake for 30 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are tender and the tops are slightly golden.