Preparation Divide watercress between 2 plates. Place chicken between sheets of waxed paper. Using rolling pin, pound chicken to even 1/2-inch thickness. Peel off paper. Sprinkle chicken with salt and pepper. Melt butter in heavy medium skillet over medium heat. Add chicken; sauté until cooked through, about 4 minutes per side. Place chicken atop watercress. Add broth, cream and mustard to skillet. Boil until sauce thickens, whisking often, about 2 minutes. Season with salt and pepper. Spoon sauce over chicken.
Preparation Divide watercress between 2 plates. Place chicken between sheets of waxed paper. Using rolling pin, pound chicken to even 1/2-inch thickness. Peel off paper. Sprinkle chicken with salt and pepper. Melt butter in heavy medium skillet over medium heat. Add chicken; sauté until cooked through, about 4 minutes per side. Place chicken atop watercress. Add broth, cream and mustard to skillet. Boil until sauce thickens, whisking often, about 2 minutes. Season with salt and pepper. Spoon sauce over chicken.