Peel and finely slice the onion. Place in a bowl and pour over enough boiling water to cover. Leave for 5 mins, then drain. Zest and juice the lemon. Pour the lemon juice over the onion with a pinch of salt. Set aside to marinate.
Trim and finely slice the leek. Heat ½ tbsp oil in a pan. Fry for 2 mins till just soft. Remove from the pan with a slotted spoon and place in a bowl.
Peel and chop your parsnips into 2 cm cubes. Heat 1 tbsp oil in the pan. Add the parsnips and cook for 3 mins till golden. Pour in boiling water to cover. Simmer for 5 mins till soft.
Meanwhile, make your slaw. Slice the cabbage as thinly as possible – use a mandoline if you have one. Trim and thinly slice the fennel. Add the cabbage, fennel, dill leaves (keep the stalks) and any fennel fronds to the onion. Stir together.
Drain the parsnips, then put them back in the pan. Cook over a medium heat for 2 mins to remove any excess moisture. Tip in the flour and mash together. Finely chop the dill stalks and add to the parsnips. Heat your oven to 100°C/Fan 80°C/Gas ¼.
Flake the salmon. Add the salmon, fried leeks and lemon zest to the mashed parsnips. Carefully fold them through – keep the salmon as chunky as possible. Taste and season. Shape the mix into 6 balls, then place on a baking tray.
Heat 1 tbsp oil in a pan. Flatten each fishcake slightly. Place into the hot pan and fry for 3 mins on each side till golden (you may need to do this in batches). Keep the cooked batches warm in the low oven while you cook the rest of the fishcakes.
Serve the fishcakes with the fennel slaw and peppery salad mix.