Heat oven to 400 degrees. Push 6 cloves garlic under skin of each chicken, 3 on each breast. Place 1/4 cup each of the chopped onions, celery and carrots, and 2 bay leaves in the cavity of each chicken. Push herbs under skin of each breast and sprinkle some of skin; put herb stems in each cavity. Rub skin of each chicken with 1 tablespoon oil; sprinkle each with 1/2 teaspoon salt and 6 turns pepper. Place carrots in a large roasting pan and top, crosswise, with celery ribs to form a bed for chickens. Surround with onions, potatoes, parsnips, and water and sprinkle with remaining 1 teaspoon salt and 12 turns black pepper. Top with chickens and roast 45 minutes. Reduce oven temperature to 350 degrees and roast until meat is falling off the bones, about 50 to 60 minutes more. Remove from oven and leave to rest, 10 minutes.