For an extra layer of flavor, marinate the chicken in a mixture of balsamic vinegar, olive oil, garlic, and herbs for at least 30 minutes before cooking. To thicken the sauce further, whisk together 1 teaspoon of cornstarch with 1 tablespoon of cold water and stir it into the simmering sauce. Cook for another minute, or until thickened. If you don't have shallots, you can substitute with yellow or white onion. Aim for a fine dice to ensure even cooking. A high-quality balsamic vinegar will make a difference in the flavor of the sauce. Look for one that is aged and has a rich, complex flavor. Consider adding a splash of heavy cream to the sauce at the end for an extra-rich and decadent finish.