Preheat your oven to 375°F (190°C).
Slice the tops off the bell peppers and carefully remove the seeds and white membranes from the inside.
In a large skillet, heat the olive oil over medium heat 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 fully browned; drain any excess liquid or fat from the pan.
Stir in the cooked brown rice, half of the tomato sauce, dried oregano, sea salt, and black pepper until well combined.
Place the bell peppers upright in a baking dish and fill each pepper cavity generously with the beef and rice mixture.
Spoon the remaining tomato sauce over the top of each filled pepper and sprinkle with the shredded sharp cheddar cheese.
Pour about a quarter cup of water into the bottom of the baking dish, cover tightly with foil, and bake for 30 minutes.
Remove the foil and continue baking for another 10 to 15 minutes until the peppers are tender and the cheese is bubbly and golden.