Preheat your oven to 375°F (190°C) and lightly grease a small baking dish.
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 sauté the diced yellow onion and minced garlic until translucent and fragrant.
Add the lean ground beef to the skillet, breaking it up with a spatula, and cook until it is completely browned.
Stir in the cooked brown rice, tomato puree, sea salt, black pepper, and dried oregano, allowing the mixture to simmer for 2 minutes.
Spoon the beef and rice mixture into the hollowed-out bell peppers, pressing down gently to pack the filling.
Place the peppers upright in the baking dish and add 2 tablespoons of water to the bottom of the dish.
Cover the dish tightly with foil and bake for 30 minutes.
Remove the foil, sprinkle the tops of the peppers with parmesan cheese, and bake for an additional 10 minutes until the peppers are tender.
Serve the warm stuffed peppers with a dollop of non-fat Greek yogurt on top for added creaminess.