In a shallow large bowl or a pie tin, whisk together the eggs and milk to make an egg batter. Place in the refrigerator until ready to use.
Place 4 slices of sandwich bread down on a cookie sheet tray lined with parchment paper, or a large serving dish or platter.
Evenly spread on the mayonnaise and Dijon mustard onto the slices of bread.
Next, evenly layer on the cheese and ham and then place on the top slice of bread.
Add the butter to a very large frying pan over low to medium heat and then dip a sandwich one at a time on all sides in the egg batter.
Place the sandwiches into the pan and cook for 3-4 minutes per side or until golden brown all around. Also, stand the sandwiches up and brown on the slice sides.
Remove sandwiches, garnish with powdered sugar and serve alongside jam or jelly.
Make-Ahead: This sandwich is meant to be eaten right away but you can keep it warm on a cookie sheet tray lined with parchment paper in the oven at 200° for 1 hour before serving it.How to Store: Cover and keep in the refrigerator for up to 4 days. You can freeze this covered for up to 3 months. Thaw in the refrigerator for 1 day before reheating and serving.How to Reheat: You can either reheat in a pan over low to medium heat with butter and cook until crisp and warm or on a cookie sheet tray lined with parchment paper in the oven at 350° for 8-10 minutes or until warm. There is an option to reheat in the microwave, but this is not recommended.Any good sandwich bread will work for this recipe. If you are looking to try something different then check out my sourdough or brioche.If gruyere cheese is not available, you can substitute it for swiss cheese.You do not want to have the pan too hot because the bread will toast but the cheese will not be melted yet.If you want to ensure your cheese is melted, then once the sandwiches are done browning place them on a cookie sheet tray lined with parchment paper and bake in the oven at 325° for 5-7 minutes.Feel free to serve this Monte Cristo with whatever jelly or jam you like.Another good way to use up some leftovers after the holidays is to load it up with some sliced turkey in addition to the ham.