Cook the potatoes in a pan of boiling salted water for 12-15 mins, or until tender enough to be easily pierced with a sharp knife. Add the green beans for the final 3 mins of cooking time, then drain well and leave to steam-dry for 5 mins. Mix the shallot with the vinegar, oil, mustard, dill and cornichons, season well and set aside.

When the potatoes have cooled slightly, halve them and gently toss with the green beans, dressing, watercress and mackerel. Divide between two plates and drizzle over any remaining dressing to serve.