Parmesan cheese helps gives this French sandwich its salty flavor.
Ingredients
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2 tablespoons unsalted butter, plus more for bread
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2 tablespoons all-purpose flour
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2 cups whole milk
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½ cup grated Gruyere cheese, plus 8 slices
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½ cup grated Parmesan cheese
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8 thick slices rustic bread
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12 slices ham
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Dijon mustard, for bread
Directions
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Melt butter in a saucepan over medium heat. Whisk in flour. Add milk; bring to a simmer. Cook, whisking frequently, until sauce thickens, about 5 minutes. Remove from heat. Add grated cheeses. Whisk until melted and smooth.
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Butter bread. Arrange half the slices, buttered side down, on a rimmed baking sheet. Top each with 2 slices Gruyere and 3 slices ham. Spread mustard onto unbuttered sides of remaining bread. Place on top of ham, buttered side up.
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Preheat broiler. Heat a grill pan or skillet over medium heat. Cook each sandwich until golden brown and cheese melts, about 5 minutes per side. Return to baking sheet. Spoon 1/3 cup sauce over top of each sandwich. Broil until tops are bubbling and golden brown, about 3 minutes.