Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and inner membranes.
In a large skillet over medium heat, add the olive oil and sauté the diced onion until translucent.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until fully browned.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for 1 minute until fragrant.
Remove the skillet from heat and stir in the cooked brown rice and tomato puree until the mixture is well combined.
Place the bell peppers upright in a baking dish and fill each one generously with the beef and rice mixture.
Sprinkle the grated parmesan cheese evenly over the top of each stuffed pepper.
Pour 0.25 cup of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the cheese is golden.