A classic French bistro sandwich with cooked ham and Gruyère between toasted sandwich bread, topped with light béchamel and more cheese before gratinating. Dijon and nutmeg add depth without distracting from the crisp bread, ham, and melted cheese.
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Grate the Gruyère, separate the cooked ham, and have 8 slices of sandwich bread ready. Preheat the oven to 220°C; having everything prepared keeps the béchamel from thickening too much while assembling.
Melt about two thirds of the butter in a medium saucepan over medium-low heat. Add the flour and cook for 1 minute, whisking. Gradually whisk in the milk and cook for 4 to 5 minutes, stirring constantly, until the béchamel is smooth and lightly coats the whisk. Add the nutmeg, white pepper, salt, and one fifth of the Gruyère; stir until melted and remove from the heat.
Spread the remaining butter very lightly over the sandwich bread, place the slices on a baking sheet, and bake for 3 to 4 minutes, just until the surface is dry and lightly golden. The bread should not become hard toast; it only needs enough structure to hold the sauce.
Spread the Dijon mustard over 4 bread slices. Divide the ham and about two fifths of the remaining Gruyère over them. Close with the other slices. Cover each sandwich with a thin layer of béchamel all the way to the edges and finish with all the remaining Gruyère.
Bake the croques at 220°C for 7 to 9 minutes, until piping hot with melted, bubbling cheese. If they need more color, move the tray closer to the top of the oven for the final minute. Serve immediately: the bread should retain crisp edges beneath a creamy, gratinated topping. And that's it! 🧡
