INGREDIENTS
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3 pounds Yukon Gold potatoes, peeled (optional) and sliced ⅛-inch thick
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2 cup shredded sharp cheddar cheese, divided
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1/2 cup grated Parmesan cheese
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3 Tablespoons butter
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1 medium onion, thinly sliced
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4 cloves garlic, minced
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3 Tablespoons all-purpose flour
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3 cups whole milk
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1 teaspoon kosher salt
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1/2 teaspoon black pepper
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1/4 teaspoon grated nutmeg
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1 1/2 Tablespoons fresh thyme, divided