Preheat the oven to Gas Mark 6/200 C/400 F. Trim any coarse stalks from the spinach, wash it well and pack into a large saucepan. Cover tightly and cook for about 5 minutes, stirring occasionally. Transfer the spinach to a blender or food processor. Add the cheese, oil, nutmeg and seasoning and mix together lightly. Transfer the mixture to a bowl and stir in the cooked lentils. Brush a 20cm (8inch) round loose-bottomed cake tin, or a flan ring set on a baking sheet, with a little of the melted butter. Line the tin with 8 sheets of the pastry, brushing lightly between each sheet with the butter. Take care to ensure that the base and sides of the tin are well covered with the pastry; allow excess pastry to overhang the sides of the tin. Place the filling in the pastry case. Top the pie with the remaining pastry sheets, brushing each with butter and tucking the ends in between the sides of the pie and the tin. Brush the top of the pie with the remaining butter and bake in the oven for 30 minutes until golden.