INGREDIENTS
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6 TBS (3/4 stick) unsalted butter, plus more to butter a six-cup gratin dish
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6 TBS all-purpose flour
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2 cups whole milk, cold
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1/2 tsp salt
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1/2 tsp freshly ground black pepper
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5 extra-large eggs, beaten
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2 1/3 cups Swiss cheese, grated (preferably Gruyère)
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3 TBS fresh chive blades, minced