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 and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it up with a spatula, and cook until fully browned.
Stir in the dried oregano, sea salt, black pepper, and half of the tomato puree, allowing the flavors to meld for 2 minutes.
Place the bell peppers upright in a baking dish and fill each cavity generously with the beef mixture.
Top each stuffed pepper with the remaining tomato puree.
Pour a splash of water into the bottom of the baking dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and the tops are slightly caramelized.