INGREDIENTS
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1 large onion, halved and thinly sliced
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2 1/2 pounds red potatoes, peeled and sliced 1/8 inch thick
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1 teaspoon salt, divided
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1 teaspoon pepper, divided
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1 cup shredded Gruyère, divided
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4 tablespoons unsalted butter, melted
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1 1/4 cups milk