Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes from the inside.
In a large skillet over medium heat, add the olive oil and diced yellow onion, sautéing until translucent.
Add the ground beef to the skillet, breaking it up with a spoon, and cook until fully browned.
Stir in the cauliflower rice, garlic powder, dried oregano, sea salt, and black pepper, cooking for 3 minutes.
Pour in half of the tomato sauce and stir to combine until the mixture is well-incorporated and moist.
Place the bell peppers upright in a baking dish and fill each one generously with the beef and cauliflower mixture.
Spoon the remaining tomato sauce over the top of each stuffed pepper to keep them juicy while roasting.
Add 0.25 cup of water to the bottom of the baking dish and cover the dish tightly with foil.
Bake for 35-40 minutes until the peppers are tender and the filling is hot throughout.