1. Pound each cutlet between two pieces of wax paper (or you can simply purchase thin cutlets).
2. In a small bowl, whisk lemon juice, zest, olive oil, mustard, garlic, thyme, rosemary, salt, pepper, and optional red pepper flakes. Place cutlets in a ziplock bag and add marinade. Put bag into a bowl to prevent leakage and place in the refrigerator overnight or for at least four hours and up to 24 hours.
3. Preheat grill to medium-high (or you can use a high-quality grill pan). When hot, place chicken cutlets on grill. Cook about six minutes on one side and then flip.
4. Begin checking the internal temperature with a meat thermometer. When the temperature reaches 165 degrees Fahrenheit (74 degrees Celsius), remove cutlets and place on a foil-lined plate. (They will continue to cook for a bit after you take them off the grill.) Cover with foil for about two minutes.
5. Slice and serve with any pan juices. Garnish with more thyme, rosemary, and optional parsley.