Cut the peppers in half top to bottom and remove the seeds and ribs. Set aside.
Heat a large pan on medium-high heat with a little olive oil. Add onions and saute until soft, about 4 minutes. Crumble in ground beef and garlic and cook until beef is no longer pink. Remove from heat and drain grease. Add salt and pepper to taste, about a teaspoon of each.
Stir rice and tomatoes into ground beef mixture.
Using a small cookie scoop, fill each pepper with about 1 tablespoons of the meat mixture. Line the stuffed peppers in a 9"x13" pan. Cover the dish with foil and bake for 20-25 minutes.
Remove the foil and top with shredded cheddar. Bake for another 5 minutes, or until cheese is completely melted.
Serve immediately.
Notes
The type of rice doesn't matter here since you are using it already cooked. So make extra or use leftover of white or brown rice. You could even use cooked quinoa!