Mostarda is not mustard. It is not sweet or acidic, not salty or spicy—at least, it’s none of those things alone. It is made mostly of fruit, but is neither jelly nor jam nor dessert. It is closest, maybe, to a relish … but what a relish! It’s confident stuff, best served with meats—its traditional counterpart is bollito misto, an assortment of boiled cuts—or cheeses that can take its sharpness.