Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and inner membranes.
Heat the olive oil in a large skillet over medium heat and sauté the diced yellow onion until it becomes translucent and fragrant.
Add the ground beef to the skillet, breaking it apart with a wooden spoon, and cook until completely browned.
Stir in the minced garlic, dried oregano, sea salt, and black pepper, cooking for one additional minute.
Fold in the cooked brown rice and half of the tomato sauce, stirring until the mixture is well combined and heated through.
Stand the bell peppers upright in a baking dish and fill each one generously with the beef and rice mixture.
Spoon the remaining tomato sauce over the top of each stuffed pepper to create a glaze.
Pour 0.25 cup of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and bake for another 10 minutes until the peppers are tender and the tops are slightly caramelized.