A Croque Monsieur is basically a toasted ham and cheese sandwich, however, the cheese is replaced with a cheese sauce, which allows you to add a range of different flavours, resulting in a much smoother texture. The technical term for this cheese sauce is a Welsh Rarebit.
40g unsalted butter
50g plain flour
250ml hot milk
250g Gruyere cheese, grated
1 tablespoon of Dijon mustard
1.5 teaspoons of Worcestershire sauce
1 Egg yolk
Melt the butter in a pan, add flour, combine and cook for about 2 minutes (called a roux), add the milk in two batches. The mixture will become thick quickly, using a whisk, continually beat until smooth. Remove from the heat and adding the cheese, mustard, Worcestershire sauce, yolk, season with salt and pepper. Pour in a container, placing a small sheet of baking paper over the top.
Butter two slices of bread, then smear the cooled Welsh Rarebit over a slice of bread, then place the desired amount of sliced ham on top. Place into a toasted sandwich machine and cook.