Use a sharp knife to cut each chicken breast in half horizontally. Arrange the 8 pieces of chicken on a foil-lined grill pan with the tomatoes. Season and spray with oil.
Cook the chicken under a preheated hot grill for about 2 minutes or until it is just cooked and still succulent. Set aside to cool, then cut the chicken into bite-sized pieces.
Meanwhile, steam or boil the potatoes. Set aside to cool, then slice into rounds.
Make the dressing by mixing together all the ingredients.
Combine the chicken, spinach, tomatoes and potatoes in a serving dish. Just before serving, pour over the dressing.