Preheat the oven to 200°C, fan 180°C, gas 6. Put the potatoes in a pan of cold water, bring to the boil then simmer for 10-15 minutes until just tender. Drain, then run under cold water until cool enough to handle.
Grate the potatoes into a bowl with the grated apples, lemon zest, the egg and flour. Mix well, season with black pepper, then shape into 12 flattened rounds.
Heat the oil in a pan and cook the potato rosti for 3-4 minutes on each side. You may need to do this in batches. Place on a baking tray and bake for 10 minutes.
Toss the flaked mackerel, crumbled cheese, toasted pine nuts and watercress together and drizzle with the lemon juice. Serve alongside the hot potato cakes.