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