Add flour and whisk until a thick creamy paste form. The mixture will begin to smell nutty.
Slowly add the milk and keep whisking continuously to avoid lumps.
Cook the sauce until the texture is thick and creamy. It should coat the back of a spoon and hold its shape when you run a finger down it.
Remove from stove top and add mustard, nutmeg, and salt.
Preheat the oven on 400F degrees.
Place the slices of bread on a baking sheet and toast them in the oven for about 10 minutes. This step will avoid the bread to get soggy from the béchamel sauce.
Remove slices of bread from the oven, and start assembling the sandwich
Spread béchamel sauce on the bottom slices.
Continue with ham and half of the cheese.
Spread a thin layer of Maille Dijon Originale Mustard on the top slices.
Place the top slices of bread mustard side down on top off the cheese.
Top the sandwich with remaining béchamel and cheese.
Bake on preheated oven for about 15 minutes or until the cheese is bubbling.
Enjoy!
Notes
If Gruyere cheese is not available, Swiss cheese can be substituted.