INGREDIENTS
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2 tablespoons unsalted butter
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1 medium onion, thinly sliced
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4 cups whole milk (or 2 cups milk and 2 cups water)
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2 to 3 garlic cloves, smashed
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1 heaping teaspoon Dijon mustard (or dry mustard powder)
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2 1/2 pounds (about 6 medium) baking potatoes, peeled
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1 1/2 cups grated cheddar cheese, divided
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3/4 cup heavy cream