Step 1Beat together the mayonnaise, basil and vinegar in a bowl until smooth.
Step 2In a large bowl, combine the watercress and 2 tablespoons of the basil mayonnaise. Toss until the leaves are lightly coated with the dressing. Divide evenly among 6 chilled plates.
Step 3To the same bowl, add the beans, chicken, tomatoes and 1/4 cup of the dressing. Season to taste with salt. Stir to combine well so all the pieces are very lightly coated with the dressing. Divide evenly and place atop the watercress. Garnish with sliced basil.
Step 4Add a teaspoon or so of dressing atop each salad and pass the remainder at the table. Serve immediately.