Rinse chicken with cold water; pat dry with a paper towel. Place chicken, breast side up, on a rack in a shallow foil-lined roasting pan or broiler pan. Starting at neck cavity of chicken, loosen skin from breast and drumsticks by inserting fingers between skin and meat and pushing gently. Combine mustard, garlic, and rosemary; rub under loosened skin, and rub over breast and drumsticks.
Bake at 375° for 1 hour and 10 minutes or until internal temperature of thigh is 175°. Transfer the chicken to a carving board; let stand 5-10 minutes (temperature will rise to 180°). Carve chicken; transfer to serving plates.