Sprinkle beef with salt, paprika and pepper. In large skillet, brown beef in butter. Place beef in large slow cooker. In same skillet, saute mushrooms, onion and garlic until tender. Transfer to slow cooker. Stir in soup, au jus mix, Worcestershire sauce and 1/4 cup water. Cover and cook on low 6 hours.
Combine remaining water and flour until smooth; add to slow cooker. Stir in sour cream. Cover and cook 1 hour longer. Serve over noodles; sprinkle with parsley.