The peppery bite of watercress contrastswith the Charentais melonsthat serve as salad bowlshere (these grapefruit-sizemelons are like cantaloupe,only sweeter).
Preheat oven to 325 degrees. Arrange prosciutto in one layer on a wire rack set over a rimmed baking sheet. Bake until crisp, about 20 minutes. Let cool. Coarsely chop half the prosciutto; set aside.
Halve melons, and discard seeds. Drizzle 3 tablespoons vinaigrette over watercress, onion, tomatoes, chopped prosciutto, and basil in a bowl. Season with salt and pepper, and toss well. Divide salad among melon halves, and serve with remaining prosciutto slices.