Place beef, mushrooms and onion in slow cooker. Mix seasoning and water until blended. Pour over beef and vegetables, toss to coat well. Cover.
Cook 8 hours on low or 4 hours on high.
Mix sour cream and flour in small bowl. Gradually stir in 1/2 cup of the hot liquid from slow cooker until well blended. Cover. Cook on high 15 minutes or until slightly thickened and bubbly. Serve over hot cooked noodles if desired.