Preheat your oven to 375°F (190°C).
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 yellow onion until translucent, about 5 minutes.
Add the minced garlic and ground beef to the skillet, breaking the meat apart with a spoon and cooking until fully browned.
Drain any excess fat from the skillet, then stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper.
Simmer the mixture for 2-3 minutes until well combined and heated through.
Stand the bell peppers upright in a baking dish and fill each pepper generously with the beef and rice mixture.
Pour a small amount of water into the bottom of the baking dish to create steam, then cover the dish tightly with foil.
Bake for 30 minutes, then remove the foil and bake for an additional 5-10 minutes until the peppers are tender.