In a small bowl, combine the first five ingredients. Add 1/4 teaspoon salt and 1/4 teaspoon pepper; set aside.
Place chicken on a rack in a shallow roasting pan. Sprinkle remaining salt and pepper inside cavity; fill with onion and grapefruit wedges. With fingers, carefully loosen the skin from both sides of chicken breast. Place rosemary and marjoram sprigs under the skin. Brush chicken with oil; rub with reserved herb mixture.
Bake, uncovered, at 325 degrees for 2 1/2 to 3 hours or until a meat thermometer reads 180 degrees. Let stand for 10-15 minutes. Discard herb sprigs and contents of cavity before slicing.
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