Croque Monsieur

Sauce: Heat the butter in a saucepan over a gentle heat until just melted. Stir in the flour with a wooden spoon. Add the milk in three lots, stirring continuously, until well incorporated. Whisk it while cooking until the sauce is thickened and smooth – it should end up being thick and silky.

Add the parmesan, nutmeg and mustard and stir to combine. Season to taste with salt and pepper then refrigerate to cool.

Preheat the oven to 200°C and put an oven tray in to heat.

To assemble: Butter one side of each piece of bread. Lay 2 pieces butter-side down on a piece of baking paper. Spread each piece with 3 tablespoons of béchamel sauce and 3 pieces of ham. Top with the onion (if using) and remaining bread, butter side up. Spread with 3 more tablespoons of sauce each. Sprinkle with gruyère cheese and pop in the oven until golden and bubbling, for about 15 minutes. Serve with extra Dijon mustard if desired.