Preheat oven to 375-400F.
Wash peppers, cut off the tops, remove the seeds and reserve the tops. Season peppers on the inside with salt and pepper. Set aside.
Cook onion and garlic in olive oil for 2 minutes. Add ground beef, half of the salt and pepper, oregano and cook for 5 minutes, until the meat is browned. Add tomato sauce and cauliflower rice. Add the remaining salt and pepper. Cook for 2 minutes, stirring frequently. Taste the mixture. Add more salt if needed.
Divide stuffing between all 4 peppers. Place the tops over each pepper. Cover with foil, bake for 30-40 minutes. Uncover and bake for additional 10-30 minutes, until the peppers are soft and fully cooked. (depends on how soft you prefer them and how fast your oven can reach this doneness).
SOME READERS HAVE REPORTED THAT THIS BAKING TIME IS NOT ENOUGH FOR THEM. PLEASE BAKE FOR UP TO 1 HOUR TOTAL FOR SOFT AND MUSHY PEPPERS, THAT ERE EASY TO PEEL. EVERYONE'R TASTE IS DIFFERENT AND YOU DO NEED TO BAKE THE PEPPERS LONGER THAT I SUGGESTED FOR SOFT PEPPERS.
You can remove the tops and add cheese on top of each pepper or serve them as they are.