Clean and wash the spinach. Chop the onion. Use a big sauce pan, cook the onion and the spinach with 3 Tablespoons of olive oil and 1/2 glass of water for approximately 4 minutes, then drain the water well. Let the spinach mixture cool a bit and then add salt and pepper, the feta cheese, 1 egg and 1 egg yolk (keep the egg white of the second egg aside). Preheat the oven on 350° F. Oil a big baking tray or dish, place the first 4 leaves of the phyllo dough, always brushing each one with some olive oil. Then put some spoonfuls of the spinach mix, if needed season it with salt and pepper, add the next layer of phyllo leaves and continue like that until the spinach mixture is finished and there are 3 leaves of phyllo dough left. Turn in at the edge and add the remaining phyllo leaves, also brushing them with olive oil. Mix the egg white with a fork until frothy and brush the top leaf, then pre-cut with a pointed knife to make the pie sections. Bake in the middle rack of the oven for about 45 minutes or until golden brown. For the last 10 minutes place it on the bottom rack of the oven. Let it cool a bit before serving.