Saute onion and garlic in olive oil until brown, add to spinach, season with pepper and oregano.Butter large oblong baking dish. Place one sheet of filo in pan, and brush with melted butter. Continue until only three or four sheets of filo are left, letting the edges of the sheets hang over the sides of the pan. Pour in the filling, and fold ends of pastry over it, brushing with more butter. Then put the remaining sheets on top, brushing with butter. Cut three or four steam vent slits in the top, down to the filling. Bake at 375 for 50 min.