Preheat oven to 400 degrees F.
In a small bowl, combine the chopped herbs, olive oil, minced garlic and lemon zest. Season with salt and pepper. Reserve.
Place the chicken, breast side up, on a large cutting board. Remove the giblets and trim excess fat. Season the chicken cavity with salt and pepper. Carefully use your fingers to loose the skin over the breasts. Using a sharp knife, make a few slits (3 or 4) on each thigh to allow the flavors to penetrate.
Spread half of the herb mixture under the chicken skin, being careful not to tear it. Place two tablespoons of butter under the skin on each side.
Slice the top of the garlic head off, exposing the cloves. Insert the lemon halves and garlic head into the chicken's cavity.
Truss the chicken with butcher's twine, tightly closing the legs together.
Rub the chicken with the remaining herb mixture and season with salt and pepper.
Scatter the potatoes and onion wedges around the chicken. Drizzle the chicken and potatoes with more olive oil.
Roast for 1 to 1.5 hours, basting every 15 minutes with the pan juices and rotating the pan, until the chicken is golden brown and a kitchen thermometer inserted in the thickest part of the thighs reads between 160 and 165 degrees F. If, by this point, the potatoes are not yet golden brown, remove the chicken to a cutting board and place the roasting dish with the potatoes back into the oven.
Let the chicken stand for 10 minutes before carving. Discard onion wedges.
Serve with the potatoes and pan juices.