Preheat the oven to 450 degrees F. Mix the mustard, thyme, 1/2 teaspoon salt and the cayenne in a medium bowl. Add the chicken breasts and turn to coat completely; set aside. In a medium shallow bowl, combine the parmesan and panko. Dredge the chicken pieces in the panko mixture, coating evenly and heavily, and pressing the coating into the meat. Put the chicken on a rack set over a baking sheet, spray with a quick burst of cooking spray and put the sheet in the middle of the oven. Bake until the chicken is golden and cooked through, 15 to 20 minutes. Let rest 5 minutes before cutting or serving.