Core cabbage and place in large bowl. Cover cabbage with boiling water and let stand five minutes. (If cabbage is very large, repeat this process after you've stuffed half of the leaves). Combine meat, onion, rice, egg, salt, pepper and paprika. Drain cabbage and remove leaves, cutting off the thick end of the stem. Put about two tablespoons of filling on a leaf. Fold sides in and roll. Place seam side down in a slow cooker. Spread sauerkraut on top. Add tomato sauce and water. Cook on low six to eight hours. Remove rolls, blend sour cream with sauce and serve with cabbage rolls.