INGREDIENTS
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1 cup milk
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1/4-pound (1 stick) unsalted butter
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1 teaspoon kosher salt
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1/8 teaspoon freshly ground black pepper
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Pinch nutmeg
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1 cup all-purpose flour
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4 extra-large eggs
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1/2 cup grated Gruyere, plus extra for sprinkling
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1/4 cup freshly grated Parmesan
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1 egg beaten with 1 teaspoon water, for egg wash