Carefully remove the core from the cabbage and place it in a large pot of boiling water. Cook for 5-7 minutes, or until the leaves are pliable, then carefully peel off the leaves and set them aside.

In a large bowl, combine the ground beef, uncooked rice, chopped onion, 1/2 cup of tomato sauce, salt, and pepper. Mix well until all ingredients are evenly distributed.

Place a portion of the meat mixture onto each cabbage leaf. Fold the sides of the leaf over the filling, then roll it up tightly.

Arrange the rolled cabbage in a large oven-safe pot or Dutch oven. In a separate bowl, whisk together the remaining tomato sauce and water, then pour this mixture over the cabbage rolls.

Cover the pot and bake at 350°F (175°C) for 1 hour, or until the cabbage is tender and the filling is cooked through. Serve hot and enjoy!