Recipe: Nick Nairn's family fish pie

I like to experiment with different types of fish, according to what's most fresh, but my all-time favourite is a mixture of salmon, cod, sweet scallops and juicy prawns. Our bodies need a regular supply of omega-3 fatty acids – found in oily fish like salmon, fresh tuna, mackerel and herring – to keep us in tip-top condition and ready to fend off the worst that winter can throw at us.

As a fishing nation, we should also be supporting our fishermen, who risk their lives to bring us their catch, by buying, cooking and eating this special harvest from the sea. At Nairn central, we try to eat fresh fish once or twice a week.

OK, lecture over. Let's get back to the matter in hand – the perfect fish for a fish pie. Adding scallops is a real treat, especially if they are hand-dived – these cost more than the dredged sort, but are sustainable and contain less sand and grit. They can be large, so after you have checked that they are firm, fresh and a good creamy colour, you may have to slice each one in two. Now a word about the roe; personally I'm not keen – it's the slime factor. But you can add it to the fish mix or the sauce if you can't bear to throw it away, for added flavour. As for prawns, don't bother with the pre-cooked frozen Atlantic 'curled pinkies'. Instead, go for the meaty, grey and glistening, medium to large raw and peeled prawns that are available in most supermarkets now.

Once your chosen fish has been cut into bite-sized chunks, mix it with lemon juice and black pepper to maximise flavour. Next comes the white sauce, considered by many people to be old-fashioned, lumpy, and stodgy, and more associated with school canteens than gastronomy. I, however, am a huge fan. The secrets of a successful white sauce lie in using more butter than normal to give a more fluid roux – this will prevent the sauce forming granular lumps when the hot milk is added. Using equal quantities of butter and flour will require two weapons in the battle against lumps: the electric hand whisk and the sieve. Forcing a lumpy sauce through a sieve into a clean pan and then giving it a good thrash with the electric whisk will restore most lumpy disasters to perfection. White sauce is a great base for adding flavours: cheese, mustard, chopped anchovies, herbs and even cooked spinach. If you have more time, you can make a proper bchamel – the kind of sauce that is found in between the layers of a good Italian lasagne. Heat the milk until just under boiling point, then add a couple of bay leaves, a blade of mace and half a raw onion. Leave to infuse for 20 minutes, then strain and make the sauce as per the recipe.

After the sauce has been poured over the fish, top it with a buttery mash to seal and protect the delicate flavours beneath. The potato must be warm or it will be difficult to spread. Once the mash is on, plonk the whole dish on a baking tray and either cook right away or chill and cook later (bring up to room temperature before cooking). And there you have it – a feast of fish in a dish, for all the family.

Nick's family fish pie

Serves six

For the fish

400g skinned salmon fillet

400g thick, skinned cod fillet

200g fresh scallops, roes removed

200g shelled raw king prawns

freshly squeezed juice of 1 lemon

freshly ground black pepper

For a cheesy white sauce

90g butter

45g flour

600ml milk

100ml double cream (optional: if not using, add the same amount of additional milk)

50g freshly grated cheddar cheese

50g freshly grated parmesan cheese (or more cheddar)

1-3 tsp Dijon mustard

finely grated zest and juice of a lemon

350g warm mashed potato

butter

sea salt

freshly ground black pepper

Preheat the oven to 200C/fan 180C/gas mark 6. Cut the salmon and cod into large chunks and remove the tough little muscle that sometimes adheres to one side of the scallop. If the scallops are huge, cut in half around the middle. Toss the fish with a good splash of lemon juice and plenty of freshly ground black pepper. Tip into the bottom of an ovenproof dish that will leave enough room for the mashed potato.

Next, make the white sauce. Melt the butter in a small heavy saucepan, add the flour and, using a heatproof spatula or wooden spoon, stir over the heat for two to three 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 milk in two or three goes, whisking well before each addition to smooth out the lumps. Make sure it is well blended. Return to the heat and slowly bring to the boil, whisking all the time to prevent the dreaded lumps forming.

Once thickened, stir in the grated cheddar, parmesan and a heaped teaspoon or more of Dijon mustard and the lemon zest and juice to taste. Test once more, then season with salt (if needed) and pepper.

Pour the sauce directly over the fish, making sure it's completely covered, then give it a wee stir, to mix the fish and sauce together. Spoon the potato in dollops all over the surface, then use the back of the spoon to spread out the potato 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 provide a better colour, and crunchy bits. Bake for about 20 minutes, until golden brown and bubbling, then let the pie rest for five minutes before serving with a big bowl of buttered peas or broccoli.

Critical points

Always use the freshest fish for this pie. Make sure your sauce is well flavoured and lump-free.

Take time to seal the fish and sauce in the dish with the mash by spreading it out evenly right to the edges. You can even pipe the mash on the top if you feel so inclined.

Set the dish on a baking tray to catch any sauce that might bubble up and out of the dish.

This article was first published in Scotland on Sunday on 24 January, 2010