Preheat the oven to 400 degrees.
Push 6 of the garlic cloves under the skin of each chicken (3 under each breast). Place 1/2 cup each of the chopped onions, celery, and carrots into the cavity of each chicken along with 2 bay leaves.
Strip the leaves from the rosemary sprigs and chop. Push the leaves under the skin of each breast and sprinkle some over the skin. Place a rosemary stem in each cavity. Rub the skin of each chicken with 1 tablespoon of the oil and sprinkle each with 1/2 teaspoon salt and 1/2 teaspoon of the pepper.
Place the halved carrots in a large roasting pan. Place the whole celery stalks across the carrots to form a bed for the chickens. Place the quartered onions, potatoes, and parsnips around the pan. Add the water and sprinkle the vegetables with the remaining teaspoon of salt and the remaining 1/2 teaspoon pepper.
Place the chickens on top of the vegetables and roast for 45 minutes. Reduce the oven temperature to 350 degrees and continue roasting until the meat is falling off the bones, 50 to 60 minutes longer.
To serve, halve the chickens and allow one-half per person, surrounded by the vegetables.
Makes 4 main-course servings