Preheat the oven to 400F.
Push 6 cloves of the garlic under the skin of each chicken (3 under each breast). Place 1/4 cup each of the chopped onions, celery, and carrots into the cavity of each chicken, along with 2 bay leaves for each.
Strip the leaves from the rosemary sprigs and chop them. 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 of the salt and 6 turns 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 and add the water. Sprinkle the vegetables with the remaining 1 teaspoon salt and the remaining 12 turns black pepper.
Place the chickens on top of the vegetables and roast for 45 minutes. Reduce the oven temperatures to 350F and roast until the meat is falling off the bones, for about 50 to 60 minutes more.
To serve, halve the chickens and allow one-half per portion, surrounded by the vegetables.
Makes 4 main-course servings.