Prepare the Phyllo Dough: Thaw phyllo dough according to the package instructions. Once thawed, set it at room temperature for about an hour.

Sauté the Onion: Heat olive oil in a skillet over medium heat. Add chopped onion and cook for 5 minutes until softened. Remove from heat.

Prepare the Spinach Filling: For fresh spinach, wash and chop it, then sauté in a dry skillet to remove excess moisture. If using frozen spinach, squeeze out all the water. Combine spinach, sautéed onions, dill, eggs, feta, Parmesan, and salt in a mixing bowl. Stir until fully combined.

Grease the Pan: Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with melted butter.

Layer the Phyllo Dough: Unroll the phyllo dough and cover it with a damp towel to prevent drying. Lay one sheet in the baking dish and brush with melted butter. Repeat this process with 7 more sheets.

Add the Filling: Spread the spinach and feta mixture evenly over the phyllo layers.

Top with More Phyllo Layers: Add another 8 sheets of phyllo dough on top, brushing each with butter. Tuck the edges in for a neat finish.

Score the Pastry: Use a sharp knife to lightly score the top layers of phyllo into squares or triangles for easy slicing after baking.

Bake the Spanakopita: Bake for 45–50 minutes, or until the phyllo is golden brown and crispy.

Cool and Serve: Allow the pie to cool slightly, then slice along the scored lines. Serve warm or at room temperature with tzatziki or a fresh Greek salad.