Cheese and onion fish pie

Method

1. Put the cod in a non-metallic bowl, cover with the salt, then leave for 20 minutes (see Know-how). Thoroughly rinse the fish, then pat dry.
2. Meanwhile, heat a glug of oil and the butter in a flameproof casserole over a medium heat. When the butter has melted, add the onion and leeks and fry for 10 minutes or until softened. Stir in the garlic, mustard and flour, then cook for 1 minute. Add the wine and beat with a wooden spoon until smooth.
3. Slowly stir in the stock, cream and juice of ½ lemon and cook, still stirring, for 5–10 minutes until the sauce is thick enough to hold its shape (see tip). If it isn’t quite there, turn the heat to low and keep stirring. Taste and add more lemon juice if it needs it.
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4. Turn off the heat, then stir in the fish, prawns and parsley. Pour into the baking dish(es). Add the frozen peas to the baking dish and stir.
5. Meanwhile, bring a large saucepan of water up to boil, then cook the potatoes for 15-20 minutes until really tender. Drain, then return to the pan to steam dry. Add the salted butter to the potatoes and mash until smooth, then taste and season with salt and pepper.
6. In a small bowl, combine the spring onions and  mature cheddar. Spoon or pipe the mashed potato on top of the pie filling, then top with the spring onion mixture. Bake for 40-45 minutes until the topping
7. is golden and crisp and the sauce is bubbling up at the edges.