Crack the egg into a bowl and mix with the milk, making a french toast type batter. The amount you need will depend on how many sandwiches you are making. Add a dash of vanilla.
Make your ham and cheese sandwich. This is where you would add the mayo and mustard if you prefer. Some people really prefer the bite of a spicy mustard on a Monte Cristo.
Once the sandwich is assembled, dip the entire assembled sandwich into the french toast mixture. You can do it separated but them the sandwich doesn't meld as well together, in my opinion.
Flip the sandwich.
Then place on a hot griddle. You can use a frying pan but I prefer a griddle. Depending on what kind of pan you are using, you might need to spray it with cooking spray or butter.
Continue to cook like a grilled sandwich, or french toast, until the bread is golden brown and the egg is cooked.
To serve you can either eat like sandwich and just get all sticky, or add the syrup to the inside, cut it with a fork
My kids like to cut it into pieces and dip it into syrup on the side.