Peel the potatoes and cook them till tender in deep, lightly salted water. Check them every few minutes. When you can easily slide a skewer into them, remove from the heat, drain them, then beat till smooth with the butter and cream.
Season with black pepper and half of the grated parmesan, then set aside.
For the filling, warm the olive oil and butter in a shallow pan, halve or quarter the mushrooms depending on their size, then brown lightly. Remove the shells from the prawns.
Skin the haddock if the fishmonger hasn’t already done so, then cut the flesh into thick chunks. Preheat the oven to 200C/gas mark 6.
Remove the mushrooms from the pan and set aside. Lightly brown the prawns and haddock in the mushroom pan, adding a little more butter or oil if necessary. Return the mushrooms to the pan.
Stir together the creme fraiche, mustard and chopped parsley then add to the pan and mix very gently, taking care not to break up the fish. Transfer the filling to a baking dish, pile the mashed potato on top, making furrows with a fork or leaving it rough, as you wish.
Dot with a little butter, scatter the reserved parmesan over the surface, then bake for 30-35 minutes till the potato is golden and the filling is bubbling around the edge.