Its name literally means 'flame cake' because it was originally slid into a wood oven for a minute or two just to test whether the fire was hot enough to bake actual bread.
Flammekueche is a paper-thin rectangle of bread dough spread with fromage blanc or crème fraîche, then scattered with onions and lardons and baked hot and fast until the edges char. Market and festival stalls slice it into strips to eat by hand.