How to Make It
Melt the butter over low heat in a small saucepan. Add the flour, stirring with a wooden spoon until it smells nutty and is slightly browned, about 2 minutes. Slowly pour the milk into the pot and cook, whisking, until the sauce is thickened. Remove from heat and stir in 1 teaspoon salt, 1/2 teaspoon pepper, the nutmeg, and Gruyère until the cheese is melted.
Heat broiler to high with the rack 6 inches from the flame. Place the bread on a baking sheet. Lightly brush half the bread with mustard and layer on the ham. Spoon the cheese sauce over the bread and sprinkle with the Parmesan. Broil until the sauce is bubbly and lightly browned, 2 to 3 minutes. Serve warm.