Greek Spinach Pie

Chop spinach and dice onion. Melt 2 Tablespoons butter in a large frying pan and saute onions until soft. Add spinach, then turn up heat and cook until all moisture is gone. Remove mixture from heat. Melt remaining butter in a small pan. Beat eggs and mix in cheeses. Add spinach mixture; season with salt and pepper. Carefully spread the filo dough out flat and cover with a moistened dish towel so that the separate sheets don't become brittle. Butter a 13 x 9 inch baking pan and place a sheet of the dough on the bottom of the pan. Brush lightly with melted butter and continue layering sheets of dough, buttering each sheet until you have a stack of 10. The dough should just fit the pan. If it doesn't tuck in any excess dough, it doesn't have to be terribly neat. Spread the spinach mixture evenly on top and stack with another 10 sheets of dough, buttering them as before. Bake in a preheated 375 degree oven for 15 minutes; turn oven down to 350 degrees for an additional 30 minutes. If the top layer begins to get too dark, cover with brown paper. Remove from oven and let rest for 15 minutes before cutting.