A rich mixture of baking and sweet potatoes, this will soon become a favorite make-ahead party dish. Jean, the author of Process This!: New Recipes for the New Generation of Food Processors plus Dozens of Time-Saving Tips, simplified this recipe using a food processor. The slicing disk on your processor will yield quick, thin potato slices. By slicing the two kinds of potato together, they're "ready-mixed." If you don't have a food processor, slice the potatoes by hand. (It will yield a thicker slice but will work just as well.) This gratin is equally delicious with baked ham, roast beef, pork, turkey, or chicken. Prep: 15 minutes, Cook: 3 minutes, Bake: 45 minutes.