Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes. Place them upright in a baking dish.
In a large skillet over medium heat, add the olive oil and sauté the diced onion until translucent, about 3-4 minutes.
Add the minced garlic and cook for another 30 seconds until fragrant.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until browned and no longer pink.
Stir in the tomato puree, dried oregano, sea salt, and black pepper. Simmer for 5 minutes to allow the flavors to meld.
Spoon the beef mixture evenly into the prepared bell peppers.
Add a small splash of water to the bottom of the baking dish and cover tightly with foil.
Bake for 30 minutes, then remove the foil and bake for an additional 10-15 minutes until the peppers are tender.
Garnish with freshly chopped parsley before serving.