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 non-stick skillet over medium heat, sauté the diced yellow onion and minced garlic until translucent and fragrant.
Add the lean ground beef to the skillet, breaking it apart with a spatula, and cook until fully browned.
Stir in the cooked brown rice, tomato puree, dried oregano, sea salt, and black pepper, allowing the flavors to meld for 2-3 minutes.
Stand the bell peppers upright in a baking dish and spoon the beef and rice mixture into each pepper until tightly packed.
Add a splash of water to the bottom of the baking dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 5-10 minutes until the peppers are tender and the tops are slightly golden.
Garnish with freshly chopped parsley before serving.