Place sliced beef in slow cooker and stir in barbecue sauce to coat. Let heat through on low for 2 or 3 hours before serving.
When ready to serve, preheat a grill. Arrange cheese, beef with sauce, and red onions on each roll. Wrap tightly in aluminium foil and place on grill. Cook until headed through, and cheese is melted.
Top with additional barbecue sauce if needed. Slice in half and serve with additional paper towels. This is too sloppy a sandwich for weak napkins.