Slow Cooker French Dip Sandwiches 1 rump or round roast, about 3 pounds 1 large onion, halved, sliced 1 packet onion soup mix 12-oz beer French bread or sandwich rolls cheese slices, optional Add all ingredients except bread and cheese to slow cooker. Cover and cook on LOW for 8 to 9 hours. Slice the beef and place on toasted split rolls or bread. With a slotted spoon, top the beef with some of the cooked sliced onions. Add a slice or two of cheese, if desired. Serve the tasty liquids for dipping.