Preheat the oven to 375°F and prepare a baking dish.
Slice the tops off the bell peppers and remove the seeds and internal membranes.
Heat the olive oil in a large skillet over medium heat and sauté the diced onion until it becomes translucent.
Add the ground beef to the skillet and cook until browned, breaking it into small crumbles with a spatula.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for one minute until the aromatics are fragrant.
Incorporate the tomato puree and cooked brown rice into the beef mixture, stirring well to combine all ingredients.
Stand the bell peppers upright in the baking dish and fill each one generously with the beef and rice mixture.
Cover the dish tightly with foil and bake for 30 minutes, then remove the foil and bake for an additional 10 minutes until the peppers are soft.
Garnish the finished peppers with chopped fresh parsley before serving.