Heat the oven to 180°C/Fan 160°C/Gas 4. Peel and chop the sweet potatoes into small chunks. Trim the leek and wash it thoroughly to remove any dirt. Finely slice it, keeping the green part and white half separate.
Heat a large pan for 1 min. Drizzle in 1 tbsp olive oil. Add the sweet potatoes and fry for 3-4 mins till slightly golden. Add the sliced green part of the leek and fry for 2 mins, stirring.
Cover the sweet potato with boiling water. Simmer for 7-8 mins till the potato is soft. Drain and steam dry in the colander for a few mins. Tip into a bowl and roughly mash.
Flake the salmon into large chunks, discarding the skin. Zest the lemon. Add to the sweet potato. Put 1 tsp flour to one side. Tip in the remaining flour, 1 tsp caraway seeds and a good pinch of salt and pepper. Gently mix together, taking care not to break the salmon up too much.
Divide the mix into 6 and shape into balls. Flatten slightly. Set a pan on a medium heat. Drizzle in 1 tbsp olive oil. Place the fishcakes into the pan and cook on each side for 3 mins till golden. You may need to do this in batches).
Place the fishcakes onto a baking tray lined with baking paper. Slide into the oven and bake for 20 mins.
While the fishcakes are cooking, peel and coarsely grate the carrots and beetroot. Use the grater attachment on a food processor if you have one. Juice in half the lemon. Add ½ tbsp olive oil and snip in most of the chives.
Finely chop half the watercress, discarding any woody stalks. Melt the butter in a small frying pan. Add the sliced white part of the leek. Fry for 2 mins on a gentle heat. Add the 1 tsp reserved flour. Squeeze in the juice from the other lemon half and add 100ml boiling water. Bring to a simmer. Remove from the heat. Season.
Pile the un-chopped watercress onto a couple of plates. Top with the carrot and beet slaw. Place x3 fishcakes onto each plate. Stir the chopped watercress through the leek and butter sauce at the last minute, then spoon onto the fishcakes to serve.