Cut the flathead and prawns into large chunks.
Heat olive oil in a heavy-based saucepan. Add onion and garlic and cook over low heat until softened. Add flour and cook, stirring, until the flour bubbles and goes grainy, about 2 minutes.
Gradually stir in stock, cream and 1¼ cups of water. Cook, stirring over low heat for 5 minutes until mixture thickens. Stir in capers and lemon zest and simmer for 2 minutes, then add flathead and simmer for 1 minute.
Add the prawns and scallops and simmer for 30 seconds more. Remove from heat, then gently stir in dill, parsley, lemon juice and season well.
Potatoes: Place kipfler potatoes in a pot of water, bring to a boil, and then simmer until soft, about 15-20 minutes. Drain well and sit in a colander to dry. Place in a bowl and crush. Add thyme, 1 tbsp oil, 1 tsp salt and generous grind of pepper. Mix together well but do not purée.
Preheat oven to 200°C.
Place the seafood filling in a large baking dish. Spread the potato mixture on top to make a rustic-looking crust, then drizzle remaining oil over the top. Put in the oven for 15-20 minutes and bake until top is golden.
