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Put the chicken in a large pan, then pour in enough cold water to just cover. Roughly chop one of the onions, drop into the pan with the carrot, tarragon and bay leaves. Cover, bring to the boil, then lower to a gentle simmer for about 1 ¾ hrs or until the chicken is cooked (the legs will fall easily away from the body). Remove from the heat, but leave the chicken in the liquid to cool for about 3 hrs. When cold, lift the chicken out of the stock then strain (skim off any fat from the stock) and keep 200ml for the sauce. Can be done a day ahead and chilled. The excess stock can be chilled the frozen.
Finely chop the second onion. Heat oil in a small saucepan, tip in the onion and fry until softened and pale golden, about 5-8 mins. Stir in the curry powder and cook for 1 min, stirring. Pour in the reserved chicken stock, then stir in the tomato purée. Cover and simmer for 10 mins. At the same time, put the apricots in a small pan with enough water to just cover them, then simmer for 15 mins. Drain, reserving 1 tbsp of the liquid. Purée the apricots in a small blender with the reserved liquid then press through a sieve (you should get about 1 tbsp purée). Remove curry sauce from the heat then stir in the sugar. Strain through a sieve, pressing as much through as you can with a wooden spoon, then stir in the lime juice and apricot purée and leave until cold.
Mix together the mayonnaise and fromage frais, then stir in the cold curry sauce. Season to taste with a good grinding of pepper.
Cut the spring onions into long, slim slivers, then set aside. If you want them to curl, put into a bowl of iced water while you finish the salad. Remove the skin from the chicken. Strip the meat off the bones in chunky pieces, remove the breasts separately and thickly slice. Gently toss the chicken with the curried sauce, the coriander and most of the mango. Scatter the watercress onto a platter. Spoon the chicken mix on top, tuck in the rest of the mango and finish with a pile of (drained) spring onions.