Preheat the oven to 350°F (175°C) and grease a 9x13 inch baking dish with melted butter. Ensure the oven is properly preheated before starting to assemble the spanakopita.

In a large mixing bowl, combine the thawed spinach, feta cheese, cottage cheese, green onions, dill, eggs, salt, and pepper, mixing thoroughly to ensure even distribution of flavors.

Brush a sheet of phyllo pastry with melted butter and lay it in the prepared baking dish. Repeat this step with half of the phyllo sheets, making sure to layer each one.

Spread the spinach and cheese mixture evenly over the layered phyllo sheets in the baking dish, using a spatula to smooth out the surface.

Layer the remaining phyllo sheets over the filling, brushing each one with melted butter, and tuck in the edges. This helps create a golden, flaky crust.

Using a sharp knife, cut through the top layers of phyllo to score the pieces into serving sizes, being careful not to cut through the filling.

Bake in the preheated oven for 45 minutes, or until the pastry is golden brown and crisp. Allow it to cool for a few minutes before serving.