Place a pot on the stove to medium heat. Add the flour and butter and mix with a hand whisk until the butter melts and it all comes together. Cook and stir for 3 minutes for the flour to cook.
Add the milk little by little stiring non stop for it to thicken.
Add in the nutmeg, season with salt and cook for 1 minute more until you get a soft sauce. Remove from the heat and let it cool down.
For the sandwiches:
Preheat the oven to 375ºF (180ºC). Place the bread slices in a baking sheet and bake for 2 minutes on each side for it to dry a little and it does not soften when adding the bechamel sauce.
Once out of the oven, spread some mustard on the slices of bread and then spread some bechamel sauce on them.
Spread about 3/4 of all the cheese between the bread slices and then place a york ham slice on top of each bread.
Pair each slice with another to assemble the sandwiches and add some more bechamel sauce on top of both sandwiches, and the remaining cheese. Put the sandwiches again on the baking sheet and put them in the oven again with the roast feature, or heat from the top for 5 minutes and they´re ready!.