1. Place onions in crockpot. Place pork atop. Combine sauerkraut, undrained tomatoes, bay leaf, paprika, caraway seed and pepper; pour over pork. Cover and cook on low-heat setting for 8 hours. Remove bay leaf. Transfer pork to a serving platter; cover to keep warm.
2. Skim fat from the surface of the sauerkraut mixture, if necessary. Pour into a large saucepan.
3. Blend cold water into cornstarch; stir into sauerkraut. Cook and stir until thickened and bubbly. Stir in sour cream. Sprinkle with paprika, if desired. Serve with pork.