Preheat the oven to 350 degrees. Arrange the chicken breast in a single layer in a shallow pan. Bring the broth to a boil. Pour enough broth in the baking pan to just cover the chicken breast. Lay a sheet of cooking parchment or aluminum foil over the chicken, and bake until cooked through, 30 minutes. Allow the chicken to cool in the liquid: then discard the liquid. Shred the chicken into bite-size pieces and place them in a large bowl. Add the grapes, pecans, celery, and chopped dill, and toss well. In a separate bowl, mix the sour cream and mayonnaise together. Toss this into the chicken salad. Season with salt and pepper. Cover and refrigerate for 2 hours. Serve on a bed of watercress or other salad greens, garnished with dill sprigs.