Preheat oven to 325°F.
Heat the olive oil in a large, deep skillet over medium heat. Once hot, add the onion and cook until softened and starting to just lightly brown, about 3-5 minutes. 2 tablespoons extra virgin olive oil, 1 medium onion
Stir in the spinach and cook until wilted.
2 pounds fresh spinach
Drain the spinach and onion mixture in a colander over the sink. Press the spinach to get as much of the liquid out of it as possible.
Return the spinach to the skillet and add the dill, parsley and lemon zest. Season with salt and pepper.
2 teaspoons dried dill, 1/4 cup fresh parsley, 1 lemon, salt, pepper
Cook until most of the liquid is gone, then stir in the feta cheese and eggs. Remove the skillet from the heat and set aside.
6 eggs, 10 ounces feta cheese
Open the pack of phyllo dough and lay it out on a countertop or sheet pan. Cover with a damp towel.
16 ounce box phyllo dough
Melt the butter.
5 tablespoons butter
Brush a very thin layer of butter onto the bottom and sides of a 9×13-inch baking dish.
Place one sheet of phyllo dough into the bottom of the pan. Brush with a very thin layer of the melted butter.
Repeat this process until approximately 3/4ths of the dough is used.
Evenly spread all of the spanakopita filling over the phyllo dough in the baking dish.
Layer the remaining phyllo dough over the top of the filling, using the same method of brushing each piece with a thin layer of butter until all of the dough is used. Fold any excess dough under. Brush the top of the pie with butter and cut three slits to allow steam to escape while baking.
Bake for approximately 1 hour or until the dough is golden brown and the filling is hot.
Cut into 12 squares and serve.