Place chicken breast halves between 2 sheets of plastic wrap; pound to 1/2-inch thickness. Sprinkle chicken with salt and black pepper. Heat a large skillet over medium-high heat. Add olive oil to pan. Add chicken; sauté 3 minutes on each side or until done. Transfer to a serving platter. Add shallots to pan; sauté for 2 minutes. Stir in chicken broth and rosemary sprig; bring to a boil. Cook 2 minutes. Stir in whipping cream; cook 2 minutes. Remove from heat, and discard rosemary. Stir in Dijon mustard. Spoon over chicken.