Preheat oven: Preheat your oven to 180°C (350°F).
Prepare the spinach: If you're using fresh spinach, steam it until wilted, then drain well and chop coarsely. If using frozen spinach, defrost and drain well.
Brown the onion: In a skillet, heat 1 tbsp. to s. of olive oil and sauté the chopped onion until translucent.
Mix the filling: In a large bowl, combine the spinach, onion, crumbled feta cheese, beaten eggs, parsley, dill, salt and pepper. Mix well to obtain a smooth filling.
Prepare the filo pastry: Unroll the filo pastry and cover it with a damp cloth to prevent it from drying out. Brush the pie dish with a little melted butter.
Assemble the spanakopita: Place a sheet of filo pastry in the pie dish, leaving it overhanging the edges. Brush with melted butter. Repeat with approximately 6 to 8 sheets of filo pastry. Spread the spinach filling over the filo pastry sheets. Cover the filling with the remaining sheets of filo pastry, brushing each sheet with melted butter.
Place in the oven: Fold the edges of the filo pastry sheets over the filling and brush the top with melted butter. Carefully cut spanakopita into squares or diamonds before baking, to make it easier to cut after cooking. Bake for 45 minutes, or until golden and crisp.
Let cool: Let the spanakopita cool slightly before serving to allow the filling to firm up.