In a 9x13 dish make the marinade by combining the zest, juice, mustard, cumin, if using, and vinegar. Spread out a sheet of plastic wrap about 2 feet long and place chicken breasts on it. Spread another sheet of plastic wrap over the chicken breasts. With flat side of a mallet, pound the chicken to an even 1/2inch thickness. Remove plastic wrap and place chicken in marinade. Cover and allow to marinate 30 min. to 1 hour. When you are ready to cook the chicken, preheat the grill. Then prepare the sauce. In a wide skillet, melt the butter over low heat and stir in the flour. Cook this roux over low heat for a minute or two, until it bubbles. Add the sugar, cinnamon, mustard, and vinegar and stir until well combined. Whisk in the orange juice, bring the heat up to medium, and stir until thickened. Lower the heat and cover the pan to keep the sauce hot while you grill the chicken. Grill the chicken just until cooked through, 3 to 5 minutes per side. Do not overcook the chicken. When serving, place the grilled chicken on a heated platter, pour some of the sauce over it, and pass the rest of the sauce. Serves 4.