EVERY JEWISH FAMILY produces a unique version of the Passover seder—the big ritual meal of traditional foods, served after and amid liturgy, storytelling, and song. We’re all surprised at each other’s customs: You eat lamb? You don’t sing “Chad Gad Ya”? And yet, virtually every seder does share a few common elements. Matzoh crumbs all over the floor. Wine stains on the tablecloth. A seder plate containing the traditional symbols of the holiday: a roasted shank bone and hardboiled egg, recalling...