"Cheesy potato casserole. Need we say more?..."
INGREDIENTS
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8 ounces Gruyère or sharp cheddar cheese, or a combination
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3 pounds Yukon gold or red potatoes
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1 small yellow onion
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2 cloves garlic
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4 tablespoons plus 1 teaspoon unsalted butter, divided
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1/4 cup all-purpose flour
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1 teaspoon Dijon mustard
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1 teaspoon kosher salt
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1/4 teaspoon freshly ground black pepper
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2 cups whole milk