For the salmon fishcakes, boil the potatoes in a pan of boiling water for 12-18 minutes, or until tender. Drain and return to the pan, then place over the heat for one minute to drive off any excess moisture. Pass the potatoes through a potato ricer into a bowl, then set aside to cool slightly.
Melt the butter in a frying pan and fry the spring onions for 2-3 minutes, or until just softened.
Stir the spring onions, parsley, lemon zest and juice into the potatoes and stir until well combined.
Season with plenty of salt and freshly ground black pepper, then add the salmon and carefully fold together.
Divide the mixture into eight small fishcakes.
Heat the oil in a frying pan and fry the fishcakes for 3-4 minutes on each side, or until cooked through and golden-brown on both sides.
For the salmon tartare, place all of the ingredients into a bowl and stir until well combined.
For the watercress mayonnaise, blanch the watercress in a pan of boiling water for one minute. Drain, refresh in iced water and then squeeze out the excess liquid.
Blend the watercress and oil in a blender until smooth.
Blend the egg yolks, vinegar, mustard, salt and lemon juice into a food processor until well combined. With the motor still running, slowly add the watercress oil until incorporated and the mixture has become mayonnaise. Season, to taste, with salt and freshly ground black pepper.
To serve, spoon the watercress onto serving plates and spread out into a circle. Spoon a layer of tartare on top, then some watercress and finally a fishcake.