Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and membranes from the inside.
In a large skillet over medium-high heat, add the olive oil and sauté the finely diced yellow onion until translucent, about 3-4 minutes.
Add the minced garlic and ground beef to the skillet, breaking the beef apart with a spatula until it is browned and cooked through.
Stir in the cauliflower rice, tomato puree, sea salt, black pepper, and dried oregano, allowing the mixture to simmer for 5 minutes until the cauliflower is slightly tender.
Place the bell peppers upright in a baking dish and fill each pepper generously with the beef and cauliflower mixture.
Pour a small amount of water into the bottom of the baking dish to create steam, then cover the dish tightly with foil.
Bake for 30 minutes, then remove the foil and bake for an additional 5-10 minutes until the peppers are soft and slightly charred on the edges.
Garnish with freshly chopped parsley before serving.