INGREDIENTS
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6 large Yukon Gold potatoes, peeled sliced 1/4-inch-thick
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3 tablespoons salted butter, plus more for the pan
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3 shallots, minced
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2 1/2 tablespoons all-purpose flour
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1 1/2 cups whole or 2% milk
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1 cup buttermilk
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2 1/2 ounces blue cheese, crumbled, divided
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2 teaspoons mustard (Dijon or brown)
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2 teaspoons Worcestershire sauce
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1 1/2 teaspoons table salt
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1/2 teaspoon freshly ground black pepper
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3/4 cup grated Parmesan cheese, divided