Begin by stripping the skin from the chicken and discarding it.
Then remove the flesh from the bones and slice it into longish, 1 inch (2.5 cm) thick pieces, and place these in a large bowl. Next, add the chopped celery to the bowl, together with the spring onions and chopped walnuts.
Now for the dressing: place the salt in a mortar and crush it quite coarsely, then add the garlic and, as you begin to crush it and it comes into contact with the salt, it will quickly break down to a puree.
Next add the tarragon, mayonnaise and yoghurt with a few twists of freshly milled pepper and blend everything together thoroughly.
Pour the dressing over the salad and toss everything together well to get a good coating of the dressing. Arrange some lettuce leaves in a large, shallow serving dish, pile the chicken salad on top, sprinkle the grapes all over, and, if you like, garnish with sprigs of watercress.
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