Preheat the oven to 400 degrees F. Melt the butter over low heat in a small saucepan and add the flour all at once, stirring with a wooden spoon for 2 minutes. Slowly pour the hot milk into the butter–flour mixture and cook, whisking constantly, until the sauce is thickened. Off the heat add the salt, pepper, nutmeg, 1/2 cup grated Gruyere, and the Parmesan and set aside. Lightly brush half the toasted croissants with mustard, add a slice of ham to each, and sprinkle with half the remaining Gruyere. Top with the other half of croissant. Slather the tops with the cheese sauce, sprinkle with the remaining Gruyere, and bake the sandwiches for 5 minutes. Turn on the broiler and broil for 3 to 5 minutes, or until the topping is bubbly and lightly browned. Serve hot.