Turn heat to 350. Remove garlic and keep warm while continuing to cook chicken, about 40 minutes more or until an instant meat thermometer inserted in thigh reads 160 degrees, or until juices run clear. Remove from oven. Pour pan juices into a fat separator or glass measuring cup while chicken rests. Skim off fat that rises to the top.
Spoon juices over the chicken.
Arrange garlic heads around chicken. Garnish with remaining rosemary sprigs. This recipe uses the Spanek vertical roaster, which is available at gourmet and cookware shops and poultry stores. For a darker skin, brush with oil before cooking, or cook an extra five to ten minutes. Carving this upright chicken is a lot easier than carving a standard chicken.