Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers, remove the seeds and membranes, and set the peppers upright in a baking dish.
In a large skillet over medium heat, add the olive oil and sauté the diced onion and minced garlic until translucent and fragrant.
Add the ground beef to the skillet, breaking it up with a spatula, and cook until it is fully browned.
Stir in the cooked rice, tomato puree, sea salt, black pepper, and dried oregano, allowing the mixture to simmer for 2-3 minutes.
Spoon the beef and rice mixture into the prepared bell peppers, pressing down gently to fill them completely.
Sprinkle the parmesan cheese over the top of each stuffed pepper.
Add a splash of water to the bottom of the baking dish, cover with foil, and bake for 25 minutes.
Remove the foil and bake for an additional 5-10 minutes until the peppers are tender and the cheese is golden.