Prepare a hot fire. Rinse chicken with cold water and pat dry; place in a glass baking dish or plastic baggie. In a small bowl, mix together garlic, rosemary, mustard, lemon juice, salt, pepper, and olive oil until well-blended. Pour over chicken breasts and turn to coat well. Cover dish or zip bag and marinate at room temperature, turning once or twice, for 30 minutes.
Remove chicken breasts from marinade and place on an oiled grill set 4-6 inches from coals. Grill chicken, turning once and basting with reserved marinade, until chicken is white throughout but still juicy, 8-10 minutes.
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