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 medium skillet over medium heat, add the olive oil and sauté the diced yellow onion until translucent, about 4 minutes.
Add the minced garlic to the skillet and cook for another 60 seconds until fragrant.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until browned and cooked through.
Stir in the dried oregano, sea salt, black pepper, and half of the tomato puree into the beef mixture.
Stand the bell peppers upright in a small baking dish and fill each pepper generously with the beef mixture.
Spoon the remaining tomato puree over the top of the beef filling in each pepper.
Add a 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.