Preheat the oven to 350°F (180°C), or 160°C if using a fan oven.
In a large skillet over medium heat. Heat 2 tablespoons of olive oil, and sautee the yellow onion until soft (3 minutes) then add green onions, dill, parsley, and garlic, and cook for another minute. Remove onto a bowl.
In the same pan, in batches add and wilt the spinach with a little olive oil or a splash of water. Once wilted, remove from the skillet and thoroughly squeeze out excess moisture.
Combine the spinach with the onion mixture in a large bowl. Add the eggs, sour cream, crumbled feta, salt, and pepper, and mix well.
Brush the bottom and sides of a 9x13 inch (23x33 cm) baking pan with olive oil. Trim the filo pastry to the size of the pan if needed.
Layer half of the filo pastry, brushing each with olive oil before adding the next. Cover the filo pastry with a damp towel until you use it all up so it does not dry out.
Spread the spinach mixture evenly over the filo pastry. Top with the remaining sheets of filo, brushing each with olive oil.
With a sharp knife, cut the pie into 12 pieces cutting all the way through.
If using, brush the top with egg wash and sprinkle with sesame seeds.
Bake for 45 minutes, or until the pastry is golden and crisp. Allow to cool slightly before serving.