Preheat your oven to 375°F (190°C) and lightly grease a baking dish with a small amount of oil.
Slice the tops off the bell peppers, removing the stems, seeds, and internal membranes to create a hollow cavity.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion until it becomes translucent and soft.
Add the ground beef to the skillet, breaking it into small crumbles with a wooden spoon, and cook until it is browned and no longer pink.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for 1 minute until the spices are fragrant.
Add the tomato puree to the beef mixture and simmer for 5 minutes to allow the flavors to meld and the sauce to thicken slightly.
Stand the bell peppers upright in the prepared baking dish and fill each one generously with the beef and tomato mixture.
Cover the dish with parchment paper and foil, baking for 30 minutes until the peppers are tender.
Remove the foil and bake for an additional 10 minutes to allow the tops to brown slightly before serving hot.