Boil a large saucepan with an inch of salted water, add diced onion and whole spinach. Simmer for 12 minutes.
I use a ceramic rectangular dish (10cm X 25cm). I cut the fillo pastry in half and laid fillo on the bottom, and leaving a bit of pastry overhanging the sides.
Paste the olive oil on pastry, layering 2 sheets at a time until all pastry is gone.
Drain the spinach, chop and add to a big bowl with beaten egg and cheeses, and spice and salt to taste. Drain excess water.
Pour into the pastry-lined tin, or casserole and fold the overhanging sides to cover the filling. Rub some oil on top, and bake in a moderate oven for 25-35 minutes, or until browned and crisp. Serve with chilli sauce and sour cream if desired.