In a large skillet, brown chicken in butter/margarine for about 15 to 20 minutes or until cooked through and juices run clear. Remove from skillet and place on a warm oven-proof platter.
Preheat oven to 150 degrees F (65 degrees C).
Stir flour into skillet drippings. Add broth and deglaze skillet by stirring vigorously until flour is somewhat dissolved and liquid has the consistency of a sauce. Add cream. Simmer, stirring, over moderate heat for about 10 minutes until sauce is a little thick. Stir in mustard and heat through.
Pour mustard sauce over chicken breasts. Put platter in warm preheated oven for about 10 to 15 minutes, then serve!