Preheat the oven 190°C (170°C fan oven) Gas Mark 5.
Combine the crème fraîche with the cornflour in a large bowl. Add the remaining ingredients except the pastry and butter, season and mix well.
Transfer the mixture to a 1.5 litre (2 ½ pt) ovenproof dish.
Brush a sheet of filo pastry with butter and lay another piece on top. Cut into ¼’s, scrunch slightly and place on top of the pie. Repeat the process until the pie is covered. (any leftover filo can be frozen for a later date).
Drizzle any remaining butter over the pie, bake for 20-25 minutes until the pastry is golden and crisp.