Filo pastry — also spelled phyllo — is the thin, translucent pastry sold ready-prepared in sheets, used for strudel, baklava, spanakopita, and borek. The sheets are stretched or rolled until nearly transparent, then layered with butter or oil: the fat between the layers is what makes them shatter into crisp, golden flakes in the oven’s heat. Working with filo demands speed and care — the sheets dry out in minutes, so cooks keep the stack covered and brush each layer quickly. Commercial production does what home kitchens cannot: rolling dough to tissue thinness consistently. The pastry’s genius is its ratio — almost all surface, barely any substance — which is why a stack of buttered sheets bakes into something simultaneously rich and ethereally light. It is the pastry of empires, from Vienna to Istanbul.