1. Preheat the oven to 200ºC/400ºF/Gas 6. Grease a 23cm/9in springform cake tin (pan). Bring a pan of water to the boil, add the asparagus and blanch for 3 minutes. Drain, refresh under cold water and drain again.
2. Heat 25g/1oz/2 tbsp of the butter in a frying pan and add the onion. Cook for 3–5 minutes or until softened. Add the mushrooms and cook for 2 minutes more. Stir in the parsley and season well with salt and black pepper.
3. In a mixing bowl combine the ricotta and mascarpone cheeses. Stir in the onion mixture. Melt the remaining butter in a small pan.
4. Line the cake tin with the filo pastry sheets, brushing each layer with melted butter and leaving the edges hanging over the sides of the tin. While you are working with one filo pastry sheet, keep the rest covered with a damp, clean dishtowel so that they do not dry out.
5. Place half the ricotta mixture in the base of the filo-lined tin. Remove any remaining pin bones from the trout fillets, then arrange them in a single layer over the ricotta. Season well.
6. Top with the asparagus and the remaining ricotta mixture. Bring the overhanging edges of the pastry over the top, and brush the layers with the remaining butter.
7. Bake the pie for 25 minutes or until golden brown. Cover loosely with foil and cook for a further 15 minutes.
8. To serve, remove the pie from the tin and place it on a warmed serving plate. Serve in slices, garnished with flat leaf parsley.