FOR THE SUMAC ONIONS:
¼ cup olive oil
1 ½ pounds red onions, peeled, halved and sliced about 1-centimeter thick
Kosher salt
1 ½ tablespoons dried sumac
FOR THE GRATIN:
2 ½ pounds (skin-on) potatoes, very thinly sliced into rounds
1 ¾ pounds chard, leaves torn from stems (stems reserved for another use) and roughly shredded (about 6 packed cups)
⅔ cup roughly chopped fresh parsley, plus 2 tablespoons finely chopped parsley for garnish
6 garlic cloves, minced
2 teaspoons grated lemon zest, plus 1/4 cup juice
Kosher salt and freshly ground black pepper
8 ounces mature Cheddar, roughly grated (about 3 cups)
1 cup chicken or vegetable stock
3 tablespoons heavy cream (double cream)
FOR THE BROWN-BUTTER PINE NUTS:
2 tablespoons unsalted butter
½ cup pine nuts
Kosher salt
Heat oven to 375 degrees. Prepare the onions: Add the oil to a large, ovenproof lidded skillet and heat over medium-high. Add the onions and 1 teaspoon salt to the hot oil and cook, stirring occasionally, until softened and browned, 15 to 20 minutes. Stir in the sumac and remove from the heat. Transfer the mixture to a bowl. Prepare the gratin: Add the potatoes, shredded chard, parsley, garlic, lemon zest, 1 tablespoon salt and 1 teaspoon pepper to a large bowl and mix well to combine. Fold through three-quarters of the sumac onions and half the cheese, and then transfer everything to the skillet, smoothing out the top to even out the potato slices. In a measuring cup or bowl, combine the stock, cream and lemon juice, and pour this all over the potato mixture. Cover tightly with foil, then top with the lid. Bake for 1 hour. Remove the gratin from the oven, and remove the lid and foil. Sprinkle evenly with the remaining cheese and bake, uncovered, until golden and bubbling, 35 to 40 minutes. Remove from heat and let gratin settle for 10 minutes. Meanwhile, prepare the pine nuts: Add the butter to a medium skillet and melt over medium-high heat. Once melted, add the pine nuts and 1/4 teaspoon salt and cook, stirring occasionally, until the nuts are golden and the butter has browned, 2 to 3 minutes. Add the buttered nuts to the bowl with the remaining sumac onions. When ready to serve, stir the extra parsley into the sumac onion mixture and spoon this all over the gratin.