1. Preheat oven to 425°F. 2. In a small bowl, whisk together mustard, syrup, and vinegar. 3. Place chicken breasts into 9 x 13 baking dish. Season with salt and lots of pepper. 4. Pour mustard mixture over chicken. Make sure each breast is coated. Put some more pepper on chicken. 5. Bake for 30-40 minutes, or until meat thermometer reads 165°F. 6. Season with fresh rosemary.