Preheat the oven to 400 degrees F.
Place the celery on a roasting pan as a sort of rack and put the chicken on top.
Cut the carrots and parsnips into similar sized sticks. Cut the sweet potatoes into larger pieces as they take less time to cook than the carrots and parsnips.
Surround the chicken with the root vegetables.
Take about half of the compound butter and smear it all over the chicken. If you feel like it, get some of the butter under the skin of the chicken.
Put half a lemon or two halves into the cavity of the chicken along with the two garlic cloves.
Take the rest of the compound butter and dot the vegetables with it.
Put the pan in the preheated oven. After 30 minutes take it out and turn all the vegetables over so that they get coated in the melted butter.
After about an hour, sqeeze the juice of the remaining lemon over the chicken and vegetables for some extra flavour.
Roast for approximately 1 hour 30 minutes or until the juices run clear in the chicken. To be safe check for an internal temperature of 185 F.