Preheat the oven to 350F and coat a 9x13 pan with non stick spray.
Heat 2 tablespoon olive oil in a large pan and saute onion until golden. Add in garlic and saute for a couple of minutes.
Add in chopped spinach and cook until wilted. Turn the heat off.
Transfer the spinach mixture to a large bowl and let it cool. Drain the excess water the spinach releases.
To the spinach, add chopped parsley, dill and green onions and mix well.
Add in the eggs and feta cheese. Stir well to everything is combined.
To assemble, place one sheet of phyllo dough at the bottom of the pan, make sure it covers the sides of the pan as well.
Brush the phyllo dough with olive oil and add another sheet of phyllo.
Repeat until you have 5 sheets of phyllo dough layered.
Spread the spinach and feta mixture evenly on the phyllo dough.
Place another phyllo sheet on the spinach mixture. Brush with olive oil and top with another layer. Repeat until you have 5 to 7 sheets of phyllo dough layered.
Fold the excess phyllo dough that's hanging from the sides onto the pie. It's okay if they break or tear.
Brush the top and the sides with olive oil.
Bake in the oven for one hour until the top is golden and crispy.
Let the spanakopita cool for a few minutes and then slice it into squares.
Serve warm or at room temperature.