A good potato gratin is all about balance. You want the potatoes cooked but not too cooked and the dish slightly saucy—not soupy or dry. The most important part, however, is the crusty, nicely browned top, which is what makes it a gratin.
Game plan: This gratin can be completely made ahead. Cool to room temperature, cover with plastic wrap or aluminum foil, and refrigerate until ready to serve. Rewarm uncovered in a 350°F oven for about 30 to 35 minutes.