This is a wonderful recipe to use up holiday leftovers. You could also make this sandwich using sliced deli chicken, turkey, or corned beef. The combination of onion, mustard, meat, and cheese on a crisp roll is really delicious. You can use other types of cheese and add vegetables, such as sliced bell peppers or mushrooms. One of the nicest things about sandwiches is that they are so versatile and variable. It's almost impossible to ruin a sandwich and a baked one feels more like a meal than a cold sandwich. Serve these sandwiches for lunch or dinner any day of the week. The sandwiches can be made ahead of time and stored in the fridge until it's time to bake. Just don't wrap the foil around the sandwiches too tightly; if you do, the cheese will stick to the foil as it melts and the sandwiches will be difficult to unwrap. If you are baking the sandwiches from the refrigerator, add another 5 to 8 minutes to the baking time. After the sandwiches are baked, you could add lettuce and tomato. Just lift off the bottom half of the roll and add the cool vegetables, then put the sandwiches back together.
