INGREDIENTS
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3 tablespoons + 1 teaspoon butter, divided
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3 tablespoons flour
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1 teaspoon dry mustard
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1 teaspoon kosher salt
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2 cups milk
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1/2 teaspoon freshly ground black pepper
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1 tablespoon chopped fresh thyme leaves
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2 pounds yukon gold potatoes, peeled and thinly sliced crosswise
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1 medium onion, diced