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To make the dressing, mix all the ingredients together in a jar and shake well. Cook the potatoes in boiling salted water for 10 mins until just tender, then drain. Cook the beans in boiling salted water for 10 mins until tender, then plunge into ice water and drain.
Toss the warm potatoes and beans together with half the spring onions and parsley, and half the dressing. Spread over a platter, then scatter over the smoked trout. Scatter over the rest of the spring onions, drizzle with the remaining dressing and serve, or leave everyone to help themselves.