1. Cut 4oz (115g) of the cheese into thin slices and shred the rest.

2. Melt the butter in a medium saucepan over low heat. Whisk in the flour and let bubble without browning for 1 minute. Whisk in the milk. Simmer, whisking often, until smooth and thick. Add the shredded cheese and mustard and stir until the cheese is melted.

3. Position a broiler rack 6in (15cm) from the heat and preheat the broiler. Toast the bread slices on 1 side only. Spread the untoasted sides lightly with butter, then top 4 slices with the ham and sliced cheese. Press the remaining 4 bread slices on top, toasted sides up, and spread with the cheese mixture.

4. Broil about 2 minutes until the sauce is bubbling and golden brown. Serve at once.

Variation:

Croque Madame: To make Croque Madames, top each serving with a fried or poached egg.