Smoked haddock, leek and mustard pie: the recipe for the first cold week of the year

There is a week every autumn when the heating goes on for the first time, the kitchen windows steam up and the only sensible thing to cook is a pie. This is mine. It is not a grand fish pie with prawns and quail’s eggs and a stock made from bones; it is a Tuesday pie, built around a couple of fillets of smoked haddock, two fat leeks and a white sauce that has had a proper tablespoon of English mustard stirred into it.

The haddock matters. Buy the pale, undyed kind if you can find it: the fishmonger on my street in Cardiff gets his from Grimsby and it costs about £14 a kilo, so two fillets for a pie that feeds four comes in under £8. The dyed yellow stuff will work, but it is saltier and the sauce ends up the colour of a hi-vis jacket.

Ingredients (serves 4): 600g floury potatoes, peeled and cut into chunks; 50g butter, plus a knob for the mash; 2 large leeks, trimmed, halved lengthways and sliced; 450g undyed smoked haddock; 500ml whole milk; 1 bay leaf; 40g plain flour; 1 tbsp English mustard; 2 tsp capers, roughly chopped; a small handful of parsley, chopped; 3 tbsp milk for the mash; salt and black pepper; a little grated cheddar if you like.

Method. Put the potatoes in a pan of cold salted water, bring to the boil and cook for 15-18 minutes until a knife slips in easily. Drain, let them steam dry for a minute, then mash with the knob of butter and 3 tbsp of milk. Season well and set aside.

While the potatoes cook, lay the haddock in a wide pan, pour over the 500ml of milk, add the bay leaf and bring just to a simmer. Turn off the heat, cover and leave for 6 minutes; the fish should flake when you press it. Lift it out on to a plate, keep the milk, and when the fish is cool enough to handle break it into large pieces, discarding any skin and bones.

Melt the 50g of butter in a saucepan over a medium heat and cook the leeks with a pinch of salt for 8-10 minutes until soft and sweet but not coloured. Stir in the flour and cook for a minute, then add the poaching milk a ladle at a time, stirring, until you have a thick, smooth sauce. Simmer for 2 minutes, then take off the heat and stir in the mustard, capers and parsley. Taste: smoked fish is salty, so you may need no salt at all, but you will want plenty of pepper.

Heat the oven to 200C (180C fan). Fold the fish gently into the sauce, trying not to break it up further, and tip it into a baking dish about 20cm by 25cm. Spoon the mash on top and rough it up with a fork so there are ridges to brown; scatter over the cheddar if using. Bake for 25-30 minutes until the top is golden and the sauce is bubbling at the edges.

Serve with peas, or with wilted greens dressed with a little vinegar, and nothing else. It reheats well the next day, which is the real reason to make it.