Trim fat from steak. Cut steak into 6 serving-size pieces. In a 3-1/2- or 4-quart slow cooker, place steak, sweet pepper, and onion. In a medium bowl, stir together soup, salsa, flour, and mustard. Pour over steak and vegetables in slow cooker.
Cover and cook on low-heat setting for 9 to 10 hours or on high-heat setting for 4-1/2 to 5 hours. If desired, serve with cornbread or mashed potatoes. Makes 6 servings.