- Place the chicken breasts in a single layer in a saucepan or flameproof casserole. Cut two slices from the lemon and add them to the chicken with the bay leaf or bouquet garni, peppercorns and wine or sherry.
- Add enough cold water to cover the chicken, cover the pan, and bring the liquid to the boil. Reduce the heat and poach the chicken for 20-25 minutes until tender and the juices run clear when the breasts are pierced with a knife at their thickest point.
- Meanwhile, to make the sauce, stir together the herbs, tomato paste and yoghurt in a bowl. Squeeze the juice from the remainder of the lemon and add it, a little at a time, until the sauce has the desired sharpness and texture. Add seasoning to taste, then chill until required.
- Remove the chicken from the cooking liquid, pat dry with kitchen paper and leave to cool for 15-20 minutes, then chill if not serving at once.
- To serve, cut the cooled chicken breasts into strips and arrange them on a mixed salad. Spoon the sauce over, garnish with sprigs of chervil and serve.
Save the strained poaching liquid to use as chicken stock in other recipes. Allow it to cool, then chill and lift off any fat. Refrigerate for up to two days or freeze for up to three months.
Poaching is an effective way to reduce fat, and produces tender, moist meats especially suitable for serving cold.