Preheat the oven to 180ºC. Place the potatoes in a saucepan, cover with boiling water and cook for 15–20 minutes or until tender. When the potatoes are done, drain them well and mash with the yogurt. Set aside and keep hot.
Meanwhile, melt the margarine in a flameproof casserole dish, add the onion and cook gently for 5 minutes or until soft. Place the fish on top, pour over 1 3/4 cups of the milk and add the bay leaves and parsley. Cover and poach in the oven for 15 minutes or until the fish flakes easily.
Cook the pasta in a saucepan of boiling water for 10 minutes, or according to the packet instructions, until just al dente. Drain and set aside.
Place the cornflour and mustard in a saucepan, add the remaining milk and mix to a smooth paste. Strain the poaching milk from the fish into the saucepan, reserving the onion, and add nutmeg. Bring to the boil, stirring well all the while. Reduce the heat and simmer for 5 minutes or until thick.
Flake the fish, discarding the skin and any bones. Stir the fish, reserved onion, prawns, mushrooms, drained pasta shells and chopped parsley into the sauce, and season with pepper. Return the mixture to the casserole dish. Spoon the mashed potatoes over the fish mixture, spreading the potato evenly, right to the edge of the dish. Fork up the surface. Bake for about 20 minutes or until bubbling and browned. Serve hot.
Natural yogurt is often used as an alternative to cream. This has the advantage of helping to lower the fat content of a recipe. In addition, yogurt provides more calcium than cream.