To a large pot, add the olive oil and saute the onion until brown. Add the dill, black pepper and salt.
Chopp the spinach and mix it together with all the ingredients in the pot. Add the crumbled feta cheese and the eggs, making sure that the mixture is slightly cool so the eggs don't scramble, and mix.
Grease the baking pan. Use half the phyllo sheets on the bottom, laying them across the full surface of the pan, brushing each lightly with butter. Spread filling evenly over the phyllo and cover with the remaining sheets, brushing each one with the melted butter. Bake for about 1 hour in an oven pre-heated to 180° C(350°F).
Allow to cool before cutting. Can be served warm or at room temperature.