Preheat oven to 375°F. Cut tops off peppers and remove seeds and membranes. Place upright in a 9x13-inch baking dish.
Heat olive oil in a skillet over medium heat. Sauté onion for 5 minutes until soft.
Add ground beef and garlic. Cook until beef is browned, then drain excess fat.
Stir in cooked rice, 1/2 of the tomato sauce, Italian seasoning, salt, pepper, and 1 cup of cheese. Mix well.
Stuff the peppers generously with the beef mixture. Top with remaining tomato sauce.
Pour water into the bottom of the dish. Cover tightly with foil and bake for 30 minutes.
Uncover, sprinkle with remaining cheese, and bake for another 15-20 minutes until cheese is melted and peppers are tender.
