For dressing, puree tuna with garlic, anchovies, mayonnaise and sour cream or yogurt. Season with pepper. For salad, combine diced turkey with cooked potatoes. Mix with 1/2 of dressing. Taste for seasoning. Arrange arugula or watercress on large plate. Mound potato and turkey mixture in centre. Arrange slices of turkey on top of chopped salad. Drizzle remaining dressing. Arrange tomatoes and olives around outside of the salad and sprinkle with parsley. SERVES: 6-8