Preheat oven to 350°. In a large bowl, mix egg, spinach and cheeses. Brush a 15x10x1-in. baking pan with some of the butter., Place 1 sheet of phyllo dough in prepared pan; brush with butter. Layer with 7 additional phyllo sheets, brushing each layer. (Keep remaining phyllo covered with a damp towel to prevent it from drying out.) Spread top with spinach mixture. Top with remaining phyllo dough, brushing each sheet with butter., Freeze, covered, 30 minutes. Using a sharp knife, cut into 1-in. squares. Bake 35-45 minutes or until golden brown. Refrigerate leftovers.