- To make the prawn stock, preheat the oven to 200°C/180°C Fan/Gas 6. 
- Place the prawn shells, onion, carrot and fennel on a baking tray and scatter over the star anise, fennel seeds and tarragon. Roast in the oven for 15 minutes or until the vegetables are just tender. 
- Transfer the roasted ingredients to a saucepan and add the wine and fish stock. Bring to the boil and then reduce to a simmer for 20 minutes. Pass the stock through a sieve and set it aside. 
- To make the velouté sauce, melt the butter in a small saucepan until foaming but not browning. Stir in the flour gradually and mix until the flour is fully incorporated. Cookout until a roux is formed. Add the prawn stock a little at a time, mixing until smooth with no lumps before adding more stock. Season with the salt and pepper. Bring the mixture slowly to a boil, stirring continuously, and then simmer for 20–30 minutes or until a smooth sauce is formed. Remove from the heat and leave to cool. 
- Place the potatoes in lightly salted water and bring to a boil. Cook for 15–20 minutes, or until tender. Drain thoroughly. Add the egg yolks and nutmeg and mash together. 
- Preheat the oven to 180°C/160°C Fan/Gas 3. Lay the fish and prawns in a deep baking dish and top with the mushrooms, peas and tarragon. Pour over the velouté sauce, top with the mash and bake in the oven for 30–40 minutes.