Heat the olive oil in a large skillet over medium heat. Add the onions and sauté until translucent, about 5 minutes. Add the spinach and cook until wilted, about 5-7 minutes.
Remove the skillet from heat and let cool. Once cooled, add the feta cheese, green onions, dill, parsley, beaten eggs, salt, and pepper to the spinach mixture.
Brush a 9x13 inch baking dish with some of the melted butter.
Lay out a sheet of phyllo dough on a clean work surface and brush it with melted butter. Repeat with 5 more sheets of phyllo, stacking them on top of one another.
Spread half of the spinach mixture over the phyllo layers.
Layer 6 more sheets of phyllo on top of the spinach mixture, brushing each sheet with melted butter.
Spread the remaining spinach mixture over the phyllo layers.
Layer the remaining sheets of phyllo on top, brushing each sheet with melted butter.
Using a sharp knife, cut the Spanakopita into squares or triangles.
Bake for 35-40 minutes, or until the phyllo is golden brown and crispy.
Remove from the oven and let cool for a few minutes before serving.