Thaw spinach and squeeze out excess liquid, then chop roughly. You can use a cheese cloth or even paper towels to squeeze all the water out.
Transfer spinach to a large bowl. Add the chopped scallions, parsley, cheese, lemon juice, salt and pepper. mix to combine and set aside.
Preheat the oven to 350 degrees F. Brush a baking sheet with some of the melted butter.
Unroll the filo dough on a flat surface and keep it covered with waxed paper or a slightly damp towel so it doesn't dry out and become brittle. Melt butter in a small bowl.
Brush melted butter onto a large casserole dish and then place the first layer of filo dough on. Brush with melted butter and place the next layer on. Repeat steps until the bottom and sides of dish are covered with 8-10 layers of filo dough. Set aside and now it's time to finish the filling.
Separate the egg whites and yolks. Place whites into the bowl of a stand mixer fitted with a whisk attachment. Mix on high until egg whites form soft peaks then add the yolks in and mix.
Fold the egg mixture into the spinach mixture then spread evenly into your prepared dish.
Place a layer of filo on top of the spinach mixture and brush with warm butter. repeat until you have 8-10 layers of filo dough covering the dish, make sure to brush the top with butter as well. For the edge you can either place the filo dough so it matches up with the sides or you can let it hang out a d but while building the top up and then tuck it into the side.
Use a very sharp knife to cut the spanakopita into the desired portions. This will make serving the baked dish much easier.
Bake at 350F (177C) for about an hour until the top is a golden hue.