Put the chicken in a saucepan together with the vegetables, bay leaf, peppercorns, and stock cubes.
Add a litre of cold water, bring to a simmer and poach for one hour.
Allow to cool in the liquid.
Lift the chicken from the poaching liquid and remove the skin and bones.
Tear the flesh into bite-sized pieces. Strain the stock and add the tarragon, then measure out 750 ml stock and set aside.
Pour 60 ml of the stock into a small saucepan and sprinkle with gelatine.
Leave to sponge for five minutes. Dissolve the gelatine over a gentle heat until clear.
Add to the remaining stock and stir thoroughly.
In an oiled, chilled shallow bowl or mould, pour in enough jelly to cover the bottom by 5 mm.
Allow to set in the refrigerator.
Arrange the grapefruit segments in the bottom and up the sides of the mould.
Fill the mould with the chicken and cover completely with the remaining cool jellied stock.
Allow to set in the refigerator. Turn out onto a platter and garnish with grapefruit segments and watercress.
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