Southwestern Sweet-Potato Gratin

Preheat oven to 350 degrees. Butter a 9-by-13- inch baking dish; set aside. Toss potatoes with 1 1/2 teaspoons salt and 1/4 teaspoon pepper. Arrange half of potatoes in buttered dish, overlapping slices. Sprinkle with half of each cheese. Top with onion.

Stir chipotle (if desired) into stock; drizzle over onion. Sprinkle with half the cilantro. Top with remaining potatoes; sprinkle with remaining cheeses and cilantro. Scatter chips on top.

Cover with foil; bake 30 minutes. Remove foil; bake until very tender and top is well browned, about 30 minutes more. Let cool slightly before serving. Serve with limes, and with crema, if desired.