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 yellow onion until it becomes translucent.
Add the minced garlic and ground beef to the skillet, cooking until the beef is thoroughly browned and crumbled.
Stir in the cauliflower rice, tomato puree, sea salt, black pepper, and dried oregano, then simmer the mixture for 3 minutes.
Stand the bell peppers upright in a baking dish and fill each one tightly with the beef and cauliflower rice mixture.
Pour a small amount of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil, sprinkle the tops with crumbled feta cheese, and bake for an additional 10 minutes until the peppers are tender.