Preheat oven to 375°F (190°C).
Slice the tops off the bell peppers and remove the seeds and membranes.
In a large skillet over medium heat, cook the ground beef and diced onion until the beef is browned and the onion is translucent.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for another minute until fragrant.
Remove from heat and stir in the cooked brown rice and tomato sauce until well combined.
Stand the bell peppers upright in a baking dish and fill each one generously with the beef and rice mixture.
Pour a small amount of water into the bottom of the dish, cover with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10-15 minutes until the peppers are tender and the tops are slightly golden.
Garnish with fresh parsley before serving.