Remove the core from the fennel and slice very finely (a mandolin gives the best results). Place in a bowl, add the lemon juice and olive oil and toss. Leave to marinate for 30 minutes.
Heat the grill and cook the salmon for 7 minutes. Trim the watercress and place in a bowl. Drain and rinse the beans, add to the watercress and toss. Arrange the fennel in the middle of each plate, top with the watercress and bean salad and lay the salmon over the top. Serve at once.
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