I take no credit for this one, as it was my father who tossed these together while we were over watching football one night. If you don't have a roast ready to go, hit the deli counter and ask for good roast beef, rare, to throw in the crock pot.
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. (You could also use a toaster oven).
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.