French
Melt the butter, stir in the flour and cook 1 minute, then whisk in the milk to make a thick béchamel; season with nutmeg, salt and pepper.
Lightly toast the bread and spread each slice with a little Dijon mustard.
Layer ham and some grated Gruyère between pairs of slices to form two sandwiches.
Spread béchamel over the tops of the sandwiches and scatter with the remaining Gruyère.
Place under a hot grill for 4 to 5 minutes until the cheese is bubbling and golden.
Serve immediately while hot and melting.