Method

1. First, make the béchamel sauce. Mix the milk with the onion, mace, parsley stalks, white peppercorns and bay leaf and heat in a pan on a medium heat until it steams. Allow to stand for 10 minutes to allow the flavours to infuse and then strain the milk.

2. Melt the butter, then remove from the heat and stir in the flour to make a smooth paste. Place the pan over a medium heat and stir for about 1 minute, until the mixture starts to bubble. Remove the pan from the heat and allow to cool for about 1 minute. Gradually add the milk, about 1 tablespoon at a time, stirring vigorously and constantly to remove the lumps. When half the milk has been added and the mixture is smooth, add the remaining milk in one go.

3. Return the pan to the heat and cook over a medium heat, stirring continuously until boiling.

4. Boil for two minutes to cook the flour and make the sauce shine. Season to taste with salt and pepper. Chill before using for the Croque Monsieur.

5. To make the Croque Monsieur, butter one slice of bread lightly, place one slice of ham on top and a sprinkle of the grated cheese. Add another slice on top.

6. Mix the mustard through the sauce and spread over the bread. Cover with the rest of the cheese and heat under a pre-heated moderate grill until melted and golden on top.