While the onion is cooking, wash the spinach.
Put it in a bowl and rub lots of salt into it.
Let the spinach/salt sit for 15 minutes. This is meant to reduce the volume of spinach. An alternative to this is to barely steam the spinach (30-60 seconds).
Beat the eggs.
Crumble the feta cheese and add to the eggs. (Note that feta cheese is fairly salty.You can replace half or all of it with ricotta cheese.)
Rinse all the salt out of the spinach. Add it and the onions to the eggs/cheese and mix.
Also add some spices. The original recipes calls for salt, pepper, and oregano.I use basil.
Melt some butter (at least 2 oz .)
Butter the bottom of a 9x13 inch baking dish.
Lay the filo down one layer at a time, brushing butter on each layer.
Turn each successive layer a little so they are not all piled directly on top of each other. The filo will hang over the sides of the dish. Continue until you have 3-5 sheets of filo left.
Pour the filling into the filo. Fold the filo that hangs over the side over the filling.
Place the last pieces of filo on top, buttering each as you go.
Trim off the filo that hangs over the edge of the dish.
These last pieces serve to cover the filling.
Make two or three slits with a knife in the top layers of filo that go all the way down to the filling.
Bake at 375 for 50 minutes.