INGREDIENTS
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Tbsp. unsalted butter
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1 sprig sage, leaves removed
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6 Tbsp. (3/4 stick) unsalted butter, plus more to butter a 6-cup gratin dish
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6 Tbsp. all-purpose flour
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2 cups cold whole milk, room temp
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1/2 tsp. salt
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1/2 tsp. freshly ground black pepper
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6 eggs
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1 cup grated Swiss cheese, preferably Gruyère (about 3 ounces)
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1 cup puréed roasted butternut squash