Using a square or rectangular 2.5 quart/liter casserole dish, brush some olive oil around the base and sides of the dish. Take 1 sheet of filo, brush lightly with the olive oil and place into the base of the dish. Repeat 4 times overlapping the sheets. Place the spinach mixture evenly over the pastry. Then brush the remaining 5 filo sheets lightly with olive oil and place over the mixture, overlapping the sheets. With kitchen scissors, cut the excess pastry and tuck the pastry into the edges of the dish. Brush the top filo sheets with the melted butter.