To poach the chicken
Bring the stock to the boil, reduce the heat, add the chicken and cook at just below the boil for approx. 15 mins. Allow the chicken to cool slightly in the stock, remove the chicken, pull apart using two forks.
Combine the mustard, balsamic, oil and yoghurt, season. Set aside half of the dressing, add the chicken to the remainder of the dressing, mix.
Mix the celeriac, cucumber, blackberries and rocket with the reserved dressing, place the chicken on top.