Sweet-potato slices provide a mellow base for the strong flavors of chipotle chile and cilantro. Crushed tortilla chips and mild grated cheese form a crunchy top to this 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.
Chihuahua, Cotija, and Mexican crema give the dish authentic flavor, but Monterey Jack, French feta, and sour cream are good substitutes.