Diana Henry's roast cod with celeriac, mushrooms, leeks & dill

1 First make the purée. Put the celeriac, potato, milk and chicken stock in a large saucepan (it needs to be large because the liquids can easily boil over). Bring to the boil, then turn the heat down to a simmer and cook for 10 minutes until the vegetables are really soft. Drain, retaining all the cooking liquid.

2 Process the celeriac and potato in a blender, adding seasoning and enough of the cooking liquid (about 100ml) to make a lovely smooth purée. Add the cream and purée again. Taste for seasoning and add a squeeze of lemon if you think the purée needs it (it always just ‘heightens’ and brightens flavours). Set aside. You will need to reheat this just before serving.

3 Preheat the oven to 180°C, gas mark 4. Lightly flour the cod on both sides then heat 15g of the butter and ½ tbsp of the olive oil in a large frying pan. Cook the fillets on a medium-high heat until pale golden on both sides (you can do this bit before your guests arrive). Transfer to a roasting tray and bake in the oven for 10-12 minutes, or until just cooked through.

4 About 5 minutes before the fish is due to be ready, steam the baby leeks. It should take just 4 minutes for them to become tender. Blot them with a clean tea towel to soak up any moisture then toss them with 15g butter, the dill and some seasoning.

5 Meanwhile, heat the rest of the olive oil in a large frying pan and cook the mushrooms really briskly, over a high heat to get good colour, then turn the heat down. Mushrooms exude a lot of liquid and you need this liquid to come out and evaporate, otherwise brown juices will seep out of them on top of your fish. Add the remaining 15g butter as the mushrooms are finishing cooking and season (the butter is great for flavour). You won’t have a lot of mushrooms but you just need a spoonful for each serving.

6 Quickly reheat the purée. Spoon some on each of 6 warmed plates, put the cod on top, the leeks around the side (using as many as looks elegant, but not to overdo it) and put some mushrooms on top of each piece of fish. Finish each plate with a sprig of dill. Serve immediately.