3 tablespoons unsalted butter ( divided ) 3 cups onion ( thinly sliced , about 10 ounces ) 4 cups russet potatoes ( peeled and thinly sliced , about 1 - 1/4 pounds ) 1 cup Dubliner cheese ( shredded , 4 ounces ) 1/2 teaspoon kosher salt 1/8 teaspoon freshly ground black pepper
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