Slash each chicken breast a couple times with a sharp knife and place in a flat dish.
Mix together the lemon zest and juice with the tarragon sprigs, garlic and seasoning. Pour over the chicken and leave to marinate for 30 minutes to 1 hour.
Pour the stock and wine (if using) into the bottom part of a large steamer or a saucepan, add the tomato purée and bring to the boil. Remove the chicken from the marinade and place into the boiling liquid. Reduce the heat, and allow the chicken to cook in simmering liquid for around 12-15 minutes. The chicken can then be removed from the pan to a bowl, covered with cling film, to rest.
While the chicken is resting, place the vegetables in the steamer. Bring the cooking liquid back up to the boil, and steam the vegetables until they are tender (around 8 minutes, but this will depend on how wide your steamer is).
Divide the vegetables between 4 bowls. Slice the chicken into 3 and sit on top of the vegetables. Spoon over the hot stock and finish by scattering some chopped tarragon and lemon zest over of the chicken.
Note: You can add some halved new potatoes to this recipe if you like, by cooking in the stock after cooking the chicken. Return to the stock to heat through when steaming the vegetables.
Recipe courtesy of Tenderstem Broccoli