Preheat your oven to 375°F and lightly grease a small baking dish.
Slice the tops off the bell peppers, remove the seeds and white membranes, and set the peppers upright in the baking dish.
Heat olive oil in a skillet over medium heat and sauté the diced onion until softened, about 4 minutes.
Add the ground beef and minced garlic to the skillet, breaking the meat apart with a spoon until it is fully browned.
Stir in the cooked brown rice, tomato sauce, sea salt, black pepper, and dried oregano, simmering for 2 minutes to meld the flavors.
Spoon the beef and rice mixture into the hollowed bell peppers, pressing down gently to pack the filling.
Cover the dish tightly with foil and bake for 30 minutes.
Remove the foil, sprinkle the tops with shredded cheddar cheese, and bake for an additional 10 minutes until the peppers are tender and the cheese is bubbly.