Prepare the Spinach: If using fresh spinach, cook the leaves in a large skillet with just the water clinging to them until wilted.
Transfer to a strainer and press out all excess liquid with a spoon. Finely chop. If using frozen spinach, ensure it is completely thawed and squeeze out all moisture.
Sauté the Aromatics: In a large skillet, heat the olive oil over medium heat.
Add the diced onions and cook until golden, about 5 minutes.
Stir in the prepared spinach and heat through.
Season the Filling: Add the dill, salt, and a generous amount of black pepper.
Cook, stirring often, until all liquid has evaporated, about 3 minutes.
Combine the Filling: Scrape the spinach mixture into a large bowl.
Stir in the milk and let it cool to room temperature.
Once cool, mix in the crumbled feta, cottage cheese, and beaten eggs until well combined.
Prepare the Pan: Preheat your oven to 300°F / 150°C.
Brush the bottom and sides of a 12x7-inch baking dish with melted butter.
Layer the Bottom Crust: Lay one sheet of filo dough in the dish, pressing it into the corners (it's okay if it overlaps). Lightly brush the entire surface with melted butter.
Repeat this process, layering and buttering, until you have used 8 sheets of filo.
Add the Filling: Spread the spinach and cheese mixture evenly over the top layer of filo.
Layer the Top Crust: Place another sheet of filo over the filling, brush with butter, and repeat until the remaining 8 sheets are used, buttering each layer.
Finish and Score: Trim the overhanging filo edges to about a 1/4-inch rim.
Brush the top generously with the remaining butter.
Using a sharp knife, make shallow cuts through the top few layers to mark your serving squares. This prevents the top from shattering when you cut it later.
Bake to Perfection: Bake for 1 hour, or until the top is deeply golden and crisp.
Let the pie rest for at least 30 minutes before cutting and serving. This allows the filling to set and the flavors to meld.
Work quickly with filo dough as it dries out fast.
Keep the stack of unused sheets covered with a clean, slightly damp kitchen towel while you work.
