Trim all excess fat from steak and cut meat into 3 inch strips about 1/2 inch wide. Combine 1/2 cup flour, the salt, pepper and dry mustard; toss with steak strips to coat thoroughly. Place coated steak strips in crock pot; stir in onion rings and mushrooms. Add beef broth and wine; stir well. Cover and cook on low setting for 8-10 hours. Before serving, combine sour cream with 1/4 cup flour; stir into crock pot. Serve stroganoff over rice or noodles.