Preheat oven to 400 degrees.
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.