Mary's croque monsieur
Preheat the grill to medium. Line a baking sheet with foil.
Melt half of the butter in a small saucepan. Brush the butter over one side of each slice of bread and place the bread on the baking sheet, buttered side up. Grill for 1–2 minutes, until lightly golden.
Turn the bread over and spread the unbuttered side with mustard. Put two slices of ham on two slices of the mustard-spread bread. Scatter half of the cheese over the ham and cover with the remaining bread, toasted side up, to make two sandwiches.
Melt the remaining butter in the saucepan, whisk in the flour and heat for a few seconds. Whisk in the hot milk until thickened. Remove from the heat and add the nutmeg and 25g/1oz of the cheese. Season with salt and pepper.
Spread the white sauce over the top of both sandwiches and sprinkle with the remaining cheese. Put back under the grill for about 5 minutes, or until the cheese has melted and the top is golden brown and bubbling.
Leave for a few seconds before slicing in half, then serve hot.