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-high heat, add the olive oil and ground beef, cooking until browned and crumbled.
Add the cauliflower rice, tomato puree, garlic powder, dried oregano, sea salt, and black pepper to the skillet.
Simmer the mixture for 5-7 minutes, stirring occasionally, until the cauliflower rice is tender and the flavors are well combined.
Place the bell peppers upright in a baking dish and fill each one generously with the beef and cauliflower mixture.
Top each pepper with the crumbled feta cheese.
Pour a small amount of water into the bottom of the baking dish to create steam, then cover with foil.
Bake for 25 minutes, then remove the foil and bake for an additional 5-10 minutes until the peppers are soft and the cheese is slightly golden.