A great seasonal dish for winter....
1. Preheat the oven to 200ºC / Fan 180ºC / gas mark 6.
2. Cut the salmon and haddock into large chunks. Mix in the prawns and toss the fish with a good splash of lemon juice and freshly ground black pepper. Tip into the bottom of a large ovenproof dish and add the peas.
3. Next make the white sauce. Melt the butter in a saucepan, add the flour and, using a heatproof spatula or wooden spoon, stir over the heat for 2-3 minutes until you have a smooth bubbling paste that is beginning to turn a sandy brown and starts to smell toasty. Leaving the pan on the heat and using a wire whisk, whisk in the cold milk in two or three goes, whisking well before each addition to whisk out the lumps. Make sure it is well blended and smooth. Return to the heat and slowly bring to the boil, whisking all the time to prevent any lumps forming.
4. Once thickened, stir in the grated cheddar, Parmesan and a heaped teaspoon or more of Dijon mustard, lemon zest and lemon juice to taste. Taste, then season with salt (if needed) and pepper.
5. Pour this directly over the fish making sure it covers all of it and give it a quick stir. Spoon the warm mashed potatoes (I like Red Roosters or Maris Pipers) in dollops all over the surface then use the back of a fork to spread out the potatoes and join the dollops together. Brush the surface with melted butter then make a pattern with the prongs of a fork all over the surface – not only does this look good but roughening up the surface will give a better colour and crunchy bits.
6. Bake for about 25 minutes until golden brown and bubbling. Remove from the oven and let it rest for 5 minutes before serving with a big bowl of spinach or broccoli.