Preheat your oven to 375°F (190°C) and lightly grease a baking dish.
Slice the tops off the bell peppers and carefully remove the seeds and white membranes from the inside.
In a large skillet over medium heat, add the olive oil, diced yellow onion, and minced garlic, sautéing for 3-4 minutes until the onion is translucent.
Add the ground beef to the skillet, breaking it apart with a wooden spoon and cooking until it is fully browned.
Stir in the cauliflower rice, tomato puree, dried oregano, sea salt, and black pepper, simmering the mixture for 5 minutes to let the flavors meld.
Stand the bell peppers upright in the prepared baking dish and spoon the beef and cauliflower mixture into each pepper until tightly packed.
Cover the dish tightly with aluminum foil and bake for 25 minutes.
Remove the foil, sprinkle the tops of the peppers with parmesan cheese, and bake for an additional 10 minutes until the peppers are fork-tender and the cheese is golden.