Prepare the Cabbage: Preheat your oven to 350°F (175°C). Bring a large pot of water to a boil. Gently remove cabbage leaves and blanch them in the boiling water for 2–3 minutes until softened. Drain and set aside.
Make the Filling: In a large bowl, combine the cooked rice, egg, onion, ground beef, salt, pepper, and garlic powder. Mix until well combined.
Assemble the Rolls: Place about ¼ cup of the beef mixture onto each cabbage leaf. Fold the sides inward and roll tightly, tucking in the ends to secure.
Prepare the Sauce: In a separate bowl, mix the tomato soup, water, vinegar, and brown sugar until smooth.
Bake the Rolls: Arrange the cabbage rolls in a baking dish, layering them as needed. Pour the tomato sauce mixture over the rolls, ensuring they are fully coated. Cover the dish with foil and bake for 2 hours, occasionally basting the rolls with the sauce.
Serve: Serve the stuffed cabbage rolls warm, topped with extra sauce. Pair with crusty bread, mashed potatoes, or a dollop of sour cream.