Topping this sandwich with onions and then melting the cheddar under the broiler should warm up your roast beef enough. Or, you can wrap it tightly in foil and heat it in a low oven (about 325-F) for about 10 minutes, switching the slices around about halfway through. Heating it in the microwave is likely to overcook it, so if you do use the microwave, just do it in 20 second intervals.