Take the lampuki fillets and check for any bones, remove if any. Peel the skin and chop the fish. Cook the onion and garlic in a spoon of olive oil until soft, add the fish chunks, pancetta, olives, capers, mushrooms and pancetta. Add little wine to avoid sticking to the pan and mix gently so that not to mash the fish pieces. Remove from the heat stir in the spinach leaves and herbs. Line a greased baking dish with layers of filo pastry, brushed with olive oil. Beat the mashed potatoes with the yolk, parmesan, nutmeg egg yolk and seasoning. Spoon the fish filling on the filo pastry and cover with the mashed potatoes. Bake in a hot oven for approximately 25 minutes until golden brown. Serve with a side salad.