Remove ALL skin and fat from the chicken legs, place them in a saucepan with one of the onions, the carrot, the peppercorns and a large pinch of sea salt, cover with the water, bring to the boil and simmer for 40-45 minutes.
Halfway through this time, chop up the remaining onion and the celery and put them to soften in the olive oil in a heavy, covered pan. When they are soft and the chicken is cooked through, strain nearly all the stock from round the chicken over the onion and celery and continue simmering for another 15-20 minutes.
Strip the chicken meat from the bones and cover it with one spoonful or so of stock to keep it moist. Discard the bones and the carrot and the whole onion from the stock.
In a large mortar crush the garlic with a little sea salt. Pound the walnuts into this until they are quite fine (*). Then pound in the chicken meat, which should be quite soft. Bind this mixture with the yoghurt, season it with pepper, liquidize the celery soup and stir the chicken and nut mixture in.
Chill before eating (**).
(*) You can crush the walnuts in an electric blender first; but do not add the garlic, as the blender would have a brutalizing effect on its taste.
(**) This soup is best eaten cold, but is also good warm !
From: B. Allen, The soup book, M Papermac, ISBN 0-333-58224-1