Method

Basic Béchamel
Melt the butter in a saucepan, add the flour and stir over a low heat for 3 minutes. Slowly add the milk and mustard, stirring constantly. Leave to simmer gently until slightly reduced, stirring occasionally.

Once reduced by a third, remove from the heat, stir in the egg yolk and season with salt, pepper and nutmeg. Cover with clingfilm to prevent a skin from forming, and leave to cool.  

Croque Monsieur
Place 4 slices of the bread onto a work surface and generously spread with the béchamel sauce. Slice the cheese and place on top of the béchamel sauce, then top with the prosciutto.

Spread the remaining slices of bread with a little Dijon mustard and more béchamel. Close the sandwiches and press them down firmly.

Preheat the oven to 180 °C

Warm a large, non-stick pan over moderate heat. Add a knob of butter to the pan. Allow the butter to bubble and gently fry the sandwiches, two at a time, until golden brown on both sides (you may need to add an extra knob of butter every now and then).

Transfer the sandwiches to a baking tray and bake in the hot oven for 5-8 minutes. Serve immediately and be careful not to burn your mouth on the delicious hot, gooey, cheesy filling.

Serve with a simple salad containing a tart element such as cornichons (small pickled dill cucumbers) or crisp, hand cut chips.