Bring the stock to the boil, add the chicken breasts and lemon zest, cover and allow to infuse over a low heat for approx. 10 mins. Remove the chicken, cover and set aside. Strain the stock into a measuring cup, rinse the lemon zest in cold water, set aside for the garnish.
Heat the oil in the same pan. Sauté the kohlrabi and potatoes. Add the cress and cook for a further 2 mins. Pour in the stock, bring to the boil, reduce the heat and simmer for approx. 15 mins. Puree the soup, stir in the cream, season to taste.
Cut the reserved chicken into thin strips, add to the soup along with the spinach and reserved lemon juice, heat gently. Garnish with the reserved cress and lemon zest.