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 heat, add the diced yellow onion and cook until translucent, about 3-4 minutes.
Add the minced garlic and ground beef to the skillet, breaking the meat apart with a spatula until browned and cooked through.
Drain any excess fat from the skillet, then stir in the cooked brown rice, tomato sauce, dried oregano, sea salt, and black pepper.
Simmer the mixture for 2 minutes to allow the flavors to meld together.
Stand the bell peppers upright in a baking dish and spoon the beef and rice mixture into each pepper until tightly packed.
Add 2 tablespoons of water to the bottom of the baking dish and 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 tender.
Garnish with fresh chopped parsley before serving.