In a bowl, combine well all sauce ingredients and season with pepper. Sauce may be made 1 day ahead and kept chilled, covered. Let sauce come to room temperature and whisk before serving.
In a small mixing bowl, whisk together the vinegar, mustard, salt, and pepper. Gradually whisk in enough oil to make a smooth dressing with a balanced taste. Stir in garlic and shallots. Place the cutlets in a glass pan, pour the marinade over, cover, and refrigerate for 2 to 4 hours.
Remove the chicken from the marinade (discard the marinade). Pat the chicken dry and season with salt and pepper. Grill chicken on an oiled rack set over glowing coals 1 to 2 minutes on each side, or until just cooked through.
Serve chicken with sauce.
Recipe adapted from Gourmet Magazine