Sauté onion in saucepan with oil until soft. Add strained spinach; set aside. In larger bowl, combine cheeses and eggs. Add spinach mixture, salt and pepper; mix well. In a buttered baking pan, layer 10 to 14 leafs of filo, brushing between each filo leaf with melted butter. Spread filling mixture evenly on filo, and cover with another 10 to 14 leaves buttered the same way. Bake at 350° for 1 hour or until browns on top.