Preheat your oven to 375°F and lightly grease a small baking dish.
Slice the tops off the bell peppers, remove the seeds and white membranes, and set the peppers upright in the baking dish.
Heat the olive oil in a large skillet over medium heat, then add the diced yellow onion and minced garlic, sautéing until translucent.
Add the ground beef to the skillet, breaking it into small crumbles with a wooden spoon, and cook until fully browned.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper, allowing the mixture to simmer for 3 to 5 minutes.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to pack the filling.
Cover the baking dish with foil and bake for 30 minutes, then remove the foil and bake for an additional 5 minutes until the peppers are soft.