Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and inner membranes.
Heat the olive oil in a large skillet over medium heat and sauté the diced yellow onion until it becomes translucent.
Add the ground beef and minced garlic to the skillet, cooking until the beef is thoroughly browned and crumbled.
Stir in the cooked white rice, tomato puree, dried oregano, sea salt, and black pepper until well combined.
Spoon the beef and rice mixture evenly into the hollowed-out bell peppers.
Place the peppers upright in a baking dish, add a splash of water to the bottom of the dish, and cover tightly with foil.
Bake for 30 minutes, then remove the foil, sprinkle with crumbled feta cheese, and bake for an additional 10 minutes until the peppers are tender.