Preheat your oven to 375°F and lightly grease a small baking dish.
Slice the tops off the bell peppers and remove the seeds and internal membranes.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion until translucent and fragrant.
Add the ground beef to the skillet, breaking it apart with a spatula, and cook until completely browned.
Stir in the tomato puree, garlic powder, dried oregano, sea salt, and black pepper, then let the mixture simmer for 5 minutes.
Place the bell peppers upright in the prepared baking dish and fill each one generously with the beef and tomato mixture.
Add two tablespoons of water to the bottom of the dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 5 minutes until the peppers are tender, then garnish with fresh parsley before serving.