Preheat your oven to 375°F and lightly grease a small baking dish.
Slice the tops off the bell peppers and remove the seeds and membranes, then place them upright in the baking dish.
In a large skillet over medium heat, add the olive oil and sauté the diced yellow onion and minced garlic until they become fragrant and translucent.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until it is thoroughly browned and no longer pink.
Stir in the cooked brown rice, tomato puree, sea salt, black pepper, and dried oregano, allowing the mixture to simmer for two minutes to meld the flavors.
Spoon the beef and rice mixture into the prepared bell peppers, packing the filling down gently to ensure they are fully stuffed.
Pour two tablespoons of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and bake for an additional 10 minutes until the peppers are tender and the filling is heated through.