Preheat the oven to 375°F and prepare a small baking dish.
Slice the tops off the bell peppers and remove the seeds and membranes from the inside.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion and minced garlic until softened.
Add the ground beef to the skillet and cook until browned, breaking it apart into small crumbles with a wooden spoon.
Stir in the tomato puree, dried oregano, sea salt, and black pepper, allowing the mixture to simmer for 5 minutes.
Stand the bell peppers upright in the baking dish and spoon the beef mixture into each pepper until full.
Pour a small amount of water into the bottom of the dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for another 10 minutes until the peppers are tender and the filling is bubbling.
Garnish with chopped fresh parsley and serve warm.