For many Americans, gratin is synonymous with a beautifully browned dish containing some combination of potatoes and chicken baked with cheese and cream until a nice golden crust is formed on top. While that definition is inarguably true, in everyday French cooking, the gratin encompasses a wide range of ingredients, from fruit to fish to vegetables, and may be savory or sweet. Christophe Felder, pastry chef at the Michelin-starred Les Ambassadeurs,...