How to Make It
Preheat the oven to 350°. Butter a 4-quart glass or ceramic baking dish. Melt the butter in a large saucepan. Add the leeks and thyme and season with salt and pepper. Cover and cook over moderate heat until the leeks soften, about 5 minutes. Uncover and cook until any liquid has evaporated. Add the cream and bring to a boil, then remove from the heat.
Spread the prunes between sheets of plastic wrap and use a rolling pin or meat pounder to flatten them to a 1/4-inch thickness. Peel the sweet potatoes and slice them 1/8 inch thick. Strain the leeks, reserving the cream.
In a large bowl, toss three-fourths of the sliced sweet potatoes with the leeks and prunes and season with salt and pepper. Spread the sweet potatoes in the prepared baking dish and arrange the remaining sweet potato slices on top in a decorative pattern. Pour the reserved leek cream evenly over the sweet potatoes.
Cover the potatoes with buttered parchment paper and then foil and bake for 45 minutes. Uncover, press the sweet potato layers down slightly and bake, uncovered, for 45 minutes longer, or until the sweet potatoes are tender, bubbling and golden brown.