"If you know, you know...."
INGREDIENTS
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6 Tbsp. unsalted butter, plus more for pan
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1 small onion, finely chopped or coarsely grated
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¼ cup all-purpose flour
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2½ cups whole milk
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1 cup sour cream
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2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt
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½ tsp. freshly ground black pepper
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¼ tsp. garlic powder
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1 30-oz. package frozen shredded hash browns, thawed
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10 oz. sharp cheddar, coarsely grated (about 2½ cups), divided