Preheat oven to 400. In a small bowl add mustard, maple syrup, vinegar, salt, garlic powder, black pepper, and onion powder and whisk until smooth.
Rinse chicken breasts and pat dry. Dip chicken breasts in sauce and turn to coat. Place chicken breasts side by side in a shallow baking dish.
Pour remaining sauce over chicken. Bake uncovered for about 20 minutes, until chicken is cooked through.
When chicken is done, plate chicken and whisk corn starch into remaining sauce left in baking dish. Spoon sauce over chicken. Top with fresh or dried rosemary if desired.
Notes
Sub honey for the maple syrup for a more traditional honey mustard chicken flavor.