In a small bowl, combine butter, sage, garlic and pepper.
Carefully loosen skin from the chicken breast. Spread half of the butter mixture under the skin and rub the rest on top of the skin. Sprinkle rosemary on chicken. Place the onion inside the chicken, then tie the legs together.
Place on a rack in a roasting pan, pour broth into pan and cover with foil. Roast for 1 hour.
Uncover, roast another 40 minutes, until pierced chicken juices run clear.