Here is a wonderful gratin of thin slices of potato and fennel smothered with melted cheese. Be sure to use an earthenware cazuela or a shallow, round baking pan so that potatoes will cook and brown evenly.
About 2 hours before serving, preheat the oven to 350°F. Peel the potatoes and cut them into paper-thin slices, preferably on a mandoline. Rinse them in cold water; drain well. Slice the fennel paper-thin.