Flódni is an elaborate, multi-layer cake consisting of four different fillings—first plum jam, then walnut, apple, and poppy seed—each between sheets of pastry. It might sound like the dessert version of Turducken, but this Hungarian-Jewish treat is no fleeting trend created as a social media–friendly novelty. The roots of flódni go back to medieval France and Germany. The word itself comes from the German fladen, a flat thin cake. According to a researcher at Tel Aviv University, the earliest reference to a Jewish flat cake also called fladen dates back to…