Preheat oven to 350°F/180°C. Butter a large shallow baking pan or jelly roll pan. Place a sheet of phyllo on a clean work surface, near the baking pan. Sprinkle lightly with feta. Scrunch the phyllo together across the wide side so that it bunches up and resembles a pleated surface. Lift this carefully and place along the short side of the baking pan. Repeat with remaining phyllo and cheese, placing each scrunched piece of feta-sprinkled phyllo snugly next to the previous one until the surface of the pan is covered.

Whisk together the beaten eggs and melted, cooled butter. Pour the seltzer over the surface of the phyllo, then pour the egg-butter mixture on top and finally sprinkle with kefalotyri. Bake until golden brown, about 50 minutes to 1 hour. Remove, cool slightly, and cut into pieces. The pie will look pleated.