Filo is a paper-thin pastry of Greek origin, similar to other pastries used throughout the eastern Mediterranean. It is available in the freezer section of many well-stocked food stores and in Greek and Middle Eastern markets. Most 1-lb. boxes contain about 20 sheets, each one measuring 14 by 18 inches. For best results, let the pastry thaw very slowly, in the refrigerator if time allows. When working with filo, be sure to keep the unused sheets covered with a barely damp kitchen towel to prevent them from drying out and becoming brittle.