Directions

1. Put the potatoes, bay leaf, peppercorns and parsley stalks in a big pan of cold water. Cover with a lid, bring to the boil and cook for 15 mins until tender. Using a slotted spoon, transfer the potatoes to a colander and leave to steam-dry. Turn the heat down, add the fish and poach gently for 5 mins until it flakes easily. Tip the potatoes into a big bowl and put the fish in the colander to drain for a few mins.
2. Add the cheese, some pepper and a little salt to the potatoes and mash well. Flake in about half the fish, discarding the skin and bones, and mash in too. Flake in the remaining fish in big chunks, scatter over the sliced spring onions and gently mix together. Roll the mixture into golf-ball-sized cakes.
3. Tip the flour onto a plate and season. Tip the egg and breadcrumbs into 2 shallow bowls each. Roll each fishcake first in the flour, then the egg, then the breadcrumbs. Sit on some parchment-lined trays that fit in your fridge. Chill for at least 1 hr or up to 24 hrs.
4. Fill a deep frying pan with 1-2cm of sunflower oil, heat until shimmering, then brown a few fishcakes at a time, turning regularly. If the oil gets too crumby, change halfway through. You can serve them straight away, or cool and chill for up to 24 hrs in the fridge, then simply warm for 30 mins in an oven at 180C/160C fan/ gas 4 before the party.
5. Make the dipping sauce up to 1 hr before serving - put the roughly chopped spring onions, the parsley leaves, watercress, rapeseed oil, 2 tbsp lemon juice and 5 tbsp water in a food processor or blender. Whizz to the consistency of single cream.
6. Pile the warm fishcakes onto a platter with a bowl of watercress sauce on the side and some lemon wedges for squeezing over, if you like.