1 pound leeks, trimmed, halved lengthwise, sliced crosswise into 1/4-inch pieces, and soaked in cold water to cover
2 tablespoons whole unsalted butter
2 tablespoons olive oil
1/4 cup garlic, minced
2 cups heavy cream
2/3 cup dried currants or diced pitted prunes
1/4 cup fresh thyme
salt and black pepper to taste
1 pound sweet potatoes or garnet yams, peeled
1-1/4 pounds Idaho potatoes, peeled and reserved in cold water
1. Lift the leeks from the water in which they're soaking so any sand remains in the water. Drain the leeks in a colander. Melt the butter and olive oil in a 2-quart saucepan over medium heat and stir in the leeks and garlic; cover and lower the heat. Cook slowly, stirring occasionally, until the leeks are softened, about 5 minutes. Add the cream, bring to a simmer, and cook over low heat for 5 minutes. Stir in the currants or prunes, fresh thyme, and salt and black pepper to taste. Set aside.
2. Preheat oven to 350 degrees. Butter a heavy-bottomed 5-quart casserole. Using a Japanese mandoline or sharp knife, slice the sweet potatoes 1/4 inch thick. Do the same with the Idaho potatoes. Beginning with Idaho potatoes, arrange a layer of potatoes on the bottom of the casserole. Season lightly with salt and pepper, then spoon 2 tablespoons of leek cream over them. Continue in this way, alternating Idaho and sweet potatoes, until all are used. With your hand, or a rubber spatula, press down firmly on the potatoes; drizzle the remaining leek cream over the top layer. Bake 50 to 60 minutes, until the gratin is bubbling around the edges and a knife slides into it easily. Remove to a warm place and let rest for 15 minutes.
3. Slice the gratin into wedges or squares and serve. It may be prepared ahead and reheated in a 300-degree oven.
Reprinted from Not Afraid of Flavor: Recipes from Magnolia Grill, with the permission of UNC Press.