We weren't sure what would result from pouring an egg-and-yogurt mixture over a gratin of potatoes. The answer? A thin layer of tangy custard that just melts into the gratin. Like a much more sumptuous version of the sour-cream-topped baked potato, this dish is a welcome change of pace.
Cooks' note:
Potatoes with custard can be baked 1 day ahead and cooled completely, then chilled, covered with foil. Reheat, covered, in a 375°F oven until hot, about 30 minutes.