Put the chicken, onion, orange rind and juice, tarragon and bay leaf in a large saucepan. Pour enough water over the top of the chicken to cover it and sprinkle with salt and pepper to taste. Cover, bring to the boil, and simmer for 1–1¼ hours, until the chicken is cooked.
Lift the chicken out of the saucepan and leave to cool. Discard the bay leaf and onion. Measure the stock, then boil until it reduces to 150 ml (½ pint). Set aside to cool, then chill in the refrigerator.
When the chicken is cold, take the meat off the bones, discarding the skin. Cut the meat into bite-sized pieces and place in a bowl.
When the stock has chilled, remove the layer of fat from the top, then reheat gently to thin it. Stir in the oil, add the vinegar, and season to taste. Pour this dressing over the chicken and toss well.
Serve immediately, garnished with orange slices, mustard, cress and tarragon sprigs, if liked, or cover and chill until required. The salad may also be served on a bed of shredded iceberg lettuce.