In a small saucepan, heat the butter over low heat and add the flour. Stir for 2 minutes.
Add the hot milk and mix with a whisk until it thickens.
Remove the pan from the heat and add salt, pepper, nutmeg, 50 grams of Gruyere and grated Parmesan cheese.
Grease the slices of bread with mustard, only on one of the faces.
On the side greased with mustard, place a slice of ham and Gruyere cheese. Place the other slice on top.
Pour the previously obtained sauce over the sandwiches and sprinkle the remaining Gruyere cheese.
Put the sandwiches in the preheated oven at 200 degrees Celsius, on the top shelf, where they will stay for 5 minutes.
Serve hot with salad, potato chips and Cafe au lait.