1. Prepare the béchamel: melt the butter in a heavy-bottomed saucepan.
2. As soon as it foams, sprinkle it with flour, salt, pepper, add a pinch of nutmeg, mix.
3. Then pour in the milk, little by little (especially not all at once), stirring with a whisk each time, until the sauce thickens.
4. Turn on the oven on th. 6/180°, take the baking sheet out of the oven and line it with aluminum foil.
5. Spread 6 slices of bread in front of you, spread them with a little mustard, add a layer of béchamel, 1 slice of ham folded in 4, grated cheese, a layer of béchamel, put the other 6 slices of bread on the on top, add a thin layer of béchamel and distribute the rest of the grated cheese.
6. Arrange the croque-monsieur on the baking sheet and put them in the oven for about 10 minutes.
7. You can place them under the broiler for 3-4 minutes at the end.
