INGREDIENTS
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3 tablespoons butter, plus more for greasing
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8 slices white bread, crusts trimmed off
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10-12 slices ham
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2 tablespoons all-purpose flour
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2 cups milk, divided
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1/4 teaspoon ground nutmeg
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2 tablespoons Dijon mustard
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3 large eggs
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2 cups Gruyere cheese, grated
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1/2 cup Parmesan cheese, freshly grated
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Kosher salt and freshly ground black pepper, to taste