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; saut 3 minutes on each side or until done.
Transfer to a serving platter.
Add shallots to pan; saut 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.