Preheat oven to 375° F. Thoroughly drain thawed frozen spinach of all water by squeezing it in a cheesecloth.
Over medium heat, melt 2 tbsp butter with scallions and leeks. Cook until soft and translucent, about 3 to 5 minutes. Let cool slightly.
In a large bowl and add dill, salt, pepper, eggs, mozzarella cheese, feta cheese, spinach, and softened leeks/scallions . Fold ingredients together until fully combined, careful to not break up feta. Do not overmix.
Melt 8 tbsp of butter in sauce pan
Lightly butter a 9-inch by 13-inch baking dish. Prepare phyllo by removing from package and unfurling it. You should have around 25 to 30 sheets. As you work, cover the dough with a damp tea towel or moistened paper towel on top to prevent it from drying out.
Carefully peel off one sheet of phyllo and lay it on the bottom of pan. With pastry brush, generously spread with butter until the sheet is moistened. Continue until you have used one third of your total phyllo dough (about 10 sheets).
Spread spinach feta filling over the bottom layer of phyllo in an even layer.
Use the rest of the phyllo to build the top layer. Layer sheets one by one, overlapping unevenly to cover entire surface. Brush melted butter in between each layer. Brush top layer with twice as much butter than was done in between layers.
Using a very sharp paring knife, score six even lines lengthwise down the phyllo, about 1/2-inch deep. Now, on the horizontal, cut four even lines across.
Bake at for 40 mins, until the top is a deep golden brown. Serve at room temperature.