Wash spinach and chop very fine. (3 10 oz bags may be used in place of loose spinach). Sprinkle spinach with 1 tb. salt and allow to stand 15 minutes. Squeeze spinach to remove excess moisture. (This is an important step in making a good filling>) Combine with cheese, onion, eggs, olive oil, salt and pepper to taste. Arrange 9 filo, which have been brushed with butter, in a greased pan (10 x 15"), each filo having been brushed separately. Cover with spinach filling. Cover with 6 filo overall, brushing each with melted butter. Seal edges to retain filling. Brush top with melted butter. Bake at 350 for 45 minutes.