Photograph: Brett Stevens
Preheat the grill to high. First make a béchamel sauce: melt the butter in a pan and add the flour; cook gently for 2-3 minutes. Gradually add the milk and simmer, whisking continuously, for 2 minutes, until thickened. Remove the pan from the heat. Season, add a generous grating of nutmeg, stir in 50g of the Gruyère and set aside.
Lay the bread on a large baking sheet and lightly toast on one side under the grill.
Preheat the oven to 200°C, fan 180°C, gas 6. Spread 4 slices of toast with mustard. Add a slice of ham to each one (you will probably need to fold the slices in half), then divide about three-quarters of the remaining Gruyère among the slices. Top with the other toasted bread slices.
Spread each sandwich with a large spoonful of béchamel sauce. Sprinkle with the remaining cheese. Bake in the oven for 10-12 minutes, then turn on the grill again and grill each croque monsieur for 2-3 minutes until the cheese is golden brown.
Get ahead: make up to a day ahead. Delicious cold, or reheat at 200°C, fan 180°C, gas 6 for 7-8 minutes.