INGREDIENTS
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1/4 cup plus 2 tablespoons freshly grated Parmigiano-Reggiano cheese
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3 tablespoons unsalted butter
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3 tablespoons all-purpose flour
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1 1/4 cups heavy cream
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4 large eggs, separated, plus 3 large egg whites
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3 tablespoons dry sherry
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6 ounces Gruyère cheese, shredded (2 packed cups)
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2 tablespoons sour cream
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1 1/4 teaspoons kosher salt
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1 teaspoon Dijon mustard
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1/2 teaspoon dry mustard
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1/4 teaspoon cayenne pepper
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1/4 teaspoon cream of tartar