INGREDIENTS
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4 tablespoons (1/2 stick) unsalted butter
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4 cups chopped yellow onions (2 large)
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1/4 cup flour
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1/4 teaspoon grated nutmeg
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1 cup heavy cream
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2 cups milk
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3 pounds frozen chopped spinach, defrosted (5 (10-ounce) packages)
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1 cup freshly grated Parmesan cheese
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1 tablespoon kosher salt
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1/2 teaspoon freshly ground black pepper
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1/2 cup grated Gruyere cheese