Start by making the topping. Put the carrots and swede into a large pan of boiling water and cook for 25-30 minutes, until tender. Drain, return to the pan and mash. Allow to cool, stir in the beaten egg and yogurt, season and set aside.
Preheat the oven to 200°C/Fan180°C/Gas Mark 6.
Place the yogurt, quark, dill and parsley in a saucepan and warm gently. Add the cod and haddock and cook for 5 minutes.
Place the fish mixture in a large mixing bowl with the hard boiled eggs, prawns and lemon juice. Season and mix well. Transfer this mixture to a deep, ovenproof dish.
Spoon the vegetable topping over the surface, then ruffle with a fork. Place in the oven and bake for 20-25 minutes until the topping is lightly browned.