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Put the potatoes in a large pan of boiling water and cook for 15-20 minutes until tender. Cook the peas in another pan of boiling water for 3 minutes then drain and put in a bowl of cold water.
Drain the potatoes well and leave them to steam dry in the colander. Once cool enough to handle, cut into 1cm-thick slices.
In a pestle and mortar, crush the capers then mix with all the other dressing ingredients in a large bowl. Add the still-warm potato slices and mix gently so you don’t break up the potatoes too much. Leave for 5 minutes so they absorb the dressing flavours.
On a serving dish, mix together the peas, chicory and watercress. Mix some of the potatoes through the leaves, then spoon the rest on top.
Break the mackerel into flakes on top of the salad and garnish with little sprigs of dill.