Add the cooled spinach mixture to the feta mixture, and mix on low speed until combined.
Add the eggs and mix until no streaks of yellow remain. Store the filling in the fridge while you prepare the pan.
3 large eggs
Assembling the Spanakopita
Preheat oven to 375℉ convection (or 400℉ conventional).
Line the bottom of your rimmed half sheet pan (13 x 18) with parchment paper to prevent sticking.
Set 4 sheets of phyllo to the side and cover them with a kitchen towel followed by a damp paper towel to prevent them from drying out. These will be used for the top of the spanakopita.
From the remaining sheets of phyllo, carefully place one sheet n the bottom of the pan. Drizzle lightly with melted butter. Repeat with a second sheet of phyllo.
With the third sheet of phyllo, place it half inside the pan and allow the other half to hang over the right edge of the pan. Place the next sheet of phyllo half inside the pan and allow the other half to hang over the left edge of the pan. Lightly drizzle melted butter on the phyllo inside of the pan.
Repeat this process with two more sheets of phyllo allowing the excess to hang over the top and bottom sections of the pan. Lightly drizzle melted butter on the phyllo inside of the pan.
Repeat Steps 4 and 5 until you have used all the phyllo.
Spread the spanakopita filling evenly inside the pan with the phyllo.
Fold all of the phyllo hanging outside the pan over to cover the filling. Lightly drizzle melted butter between the layers.
Using the four reserved sheets of phyllo, crinkle one sheet at a time and place them on the top of the spanakopita until covered. This will give the spanakopita a nice look and texture.
Drizzle the remaining butter on the top of the spanakopita and score into 16 pieces.
Bake spanakopita for 45 mintues (or until the top is golden brown and flaky). Allow to rest for 10-15 mintues before serving.