Preheat the oven to 425 degrees and place a heavy roasting pan to preheat in the oven.
Season the capon cavity with salt and pepper. Put the crushed garlic, chopped onion and dry herbs inside the cavity. Using the twine, tie the legs closed. Rub the entire outside of the capon with an even coat of the olive oil. Pick the herbs from their stems, coarsely chop them, and sprinkle them evenly on the chicken. Place the chicken into the pre-heated roasting pan and roast at 425 degrees for 45 minutes. Reduce the temperature to 350 degrees and continue to roast for an additiional 1 1/2 hours, or until a meat thermometer registers 165 degrees and the juices run clear when the thigh is pierced. Allow to rest on a platter for 15 to 20 minutes before carving.
While the chicken is resting, place the roasting pan on top of the stove, remove any chicken fat with a spoon, and, over medium heat, deglaze the pan drippings with the chicken broth. Use a spoon to scrape up all the crusty bits (that's where the most concentrated flavor is) and bring to a boil. Remove from the heat and strain the liquid into a gravy boat to pass at the table.