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2 tablespoons unsalted butter
3 tablespoons all-purpose flour
2 cups hot milk
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
12 ounces Gruyere, grated (5 cups)
1/2 cup freshly grated Parmesan
8 ounces honey ham, sliced but not paper thin
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 butterflour 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.
Halve and toast the croissants, place the slices on 2 baking sheets and bake for 4 minutes. Turn each slice and bake for another 2 minutes, until toasted.
Lightly brush half the croissant with mustard, add a slice of ham to each, and sprinkle with half the remaining Gruyere. Top with another piece of toasted bread. 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.