Scalloped potatoes are a tried-and-true side dish. There’s no reason to reinvent a classic, but adding a kick, a rather strong kick, can be a fun twist. The piquant horseradish in this recipe does exactly that. It cuts through the double richness of the cream and cheese and makes you crave another serving. Swapping a single variety of potato for three — purple, sweet and russet — adds more texture and flavor. Purple potatoes (see Tip) pop on the plate and offer that earthy flavor but with a less starchy texture, and the sweet potatoes are meltingly tender. Give this festive, upgraded version of scalloped potatoes a try at your next gathering.