Boil potatoes for 12-18 minutes until tender, then drain and rice.
Melt butter in a pan and fry spring onions until softened.
Stir onions, parsley, lemon zest, and juice into the riced potatoes.
Season the mixture and fold in the diced salmon.
Shape the mixture into eight small fishcakes.
Fry fishcakes in oil for 3-4 minutes per side until golden.
Combine all tartare ingredients in a bowl.
Blanch watercress for one minute, refresh in ice water, and squeeze dry.
Blend the blanched watercress with oil until smooth.
Process yolks, vinegar, mustard, salt, and juice while slowly adding watercress oil.
Plate by layering watercress, tartare, more watercress, and a fishcake.