Roast the whole chicken breasts with the skin on and bone in to yield maximum flavor and juiciness, but remove the skin before eating for a lighter meal.
Let the chicken breasts stand at room temperature for 15 minutes. Heat oven to 400 degrees with two racks centered. Lightly spray two 14-by-12-inch roasting or jelly-roll pans with cooking spray. The potatoes may be peeled or not, as desired. If the potatoes are larger than 1 1/2 inches in diameter, cut them into 1 1/2- to 2-inch chunks.
Separate garlic head into cloves. Divide potatoes and garlic cloves between roasting pans. Drizzle potatoes and garlic in each pan with 1 tablespoon olive oil. Lightly toss potatoes and garlic until they are covered with oil. Place one whole chicken breast, skin-side up, in one pan. Place remaining two whole breasts in the other pan, skinsides up. Sprinkle ingredients in each pan with 3/4 teaspoon salt and 1/4 teaspoon pepper. Roast for 30 minutes. Rotate pans in oven, and use a spatula to turn potatoes for even browning. Continue to cook 20 to 25 minutes, until chicken skin is crisp and an instant-read thermometer inserted into the breast meat reads 180 degrees. Don't touch any bones with thermometer; bones conduct heat and will give a false reading.
To serve, divide the potatoes evenly between six plates with a few cloves of roasted garlic for each person. Use a sharp knife to remove each half of breast meat from the bone. Remove skin from each serving before eating. Garnish with rosemary sprigs.