TO PREPARE THE DRESSING: Combine about 1 tablespoon of the water with the cornstarch to dissolve. Combine the remaining water, cornstarch mixture, vinegar and tomato paste in a saucepan and bring to a boil. Continue boiling until slightly thickened. Remove from the heat and whisk in the garlic, mustard, basil, oregano, cayenne, salt, pepper and olive oil. Refrigerate at least 1 hour.
TO PREPARE THE SALAD: Scrub the potatoes but do not peel. Cut in halves. Place in a steamer basket and steam over boiling water until tender, about 20-25 minutes. Drain and tranfer to a bowl. Add 4 tablespoons of the dressing and the shallot. Refrigerate 1 to several hours.
Bring a small pan of water to the boil, add the beans and time about 5 minutes, or until tender. Drain and plunge into ice water to stop the cooking. Pat dry and add to the potatoes.
Stir in the olives, tomatoes and basil leaves. Add enough dressing to coat the salad. If making the salad ahead, refrigerate; remove from refrigeration about 1 hour before serving.