Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and internal membranes carefully.
In a large skillet over medium heat, add the olive oil and sauté the finely diced onions until they become translucent.
Add the ground beef to the skillet, breaking it up with a wooden spoon until it is fully browned and cooked through.
Stir in the minced garlic, cauliflower rice, tomato puree, dried oregano, sea salt, and black pepper.
Simmer the beef and vegetable mixture for about 5 minutes to allow the flavors to meld and the cauliflower rice to soften slightly.
Place the prepared bell peppers upright in a baking dish and fill each one to the brim with the beef and cauliflower mixture.
Sprinkle the top of each stuffed pepper with the Parmesan cheese.
Pour a small amount of water (about 0.25 cup) into the bottom of the baking dish to create steam during the cooking process.
Cover the dish tightly with foil and bake for 30 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are tender and the cheese is golden.