Roast beef sandwiches can be enjoyed both hot and cold, it all comes down to preference. Because we are using deli meat in this recipe, there's no need to reheat the beef as it's already cooked - but you certainly can if you would like to. We start this recipe off with room temperature ingredients so it isn't necessarily cold. If desired, you can toast it in skillet with butter to crisp up the bread and melt the cheese.