Drain off all but 1 tablespoon fat/juices from the skillet but leave the rosemary and garlic. Add chicken broth and scrape up any bits on the bottom of the pan; simmer, stirring occasionally for 3 minutes to reduce. Stir in heavy cream, balsamic vinegar and Dijon. Simmer until slightly thickened, about 2 minutes. If you would like more sauce, you can add more cream ;).