INGREDIENTS
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1 pound russet potato
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5 tablespoons salted butter, divided
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2 large yellow onions, sliced thin
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2 tablespoons rosemary, finely chopped
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3 tablespoons all-purpose flour
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3 cups whole milk
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1/2 teaspoon salt
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1/4 teaspoon pepper
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12 ounces (about 3 cups) Gouda cheese, grated
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8 ounces dry elbow pasta, cooked according to package directions