Preheat your oven to 375°F and lightly grease a baking dish with a small amount of oil.
Slice the tops off the bell peppers, remove the seeds and ribs, and set the peppers upright in the prepared dish.
In a large skillet over medium heat, warm the olive oil and sauté the diced yellow onion and minced garlic until soft and fragrant.
Add the ground beef to the skillet, browning it thoroughly while breaking it into small crumbles with a wooden spoon.
Stir in the tomato puree, dried oregano, sea salt, and black pepper, then let the mixture simmer for five minutes to thicken.
Spoon the beef and tomato mixture evenly into the hollowed bell peppers, pressing down slightly to pack them.
Top each stuffed pepper with the crumbled feta cheese for a salty, tangy finish.
Add two tablespoons of water to the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and bake for another 10 minutes until the peppers are tender and the cheese is lightly browned.