Preheat oven to 375 degrees. Finely chop the leaves of one sprig rosemary and one clove garlic. In a small bowl, combine mustard, chopped rosemary, and chopped garlic; set aside.
Rub each chicken inside and out with half a lemon. Place a lemon half in each cavity. Add to the cavity of each chicken a stalk of celery, two onion quarters, a sprig of rosemary, and a clove of garlic. Rub chickens all over with mustard mixture, and season with salt and pepper. Place on a rack in a large roasting pan. Sprinkle all over with wine. Pour remaining wine into roasting pan.
Bake, basting about every 15 minutes, until juices run clear, 1 1/2 to 1 3/4 hours. Add water or wine as additional basting juices are needed. Remove from oven, transfer chickens to a platter, and let stand 10 minutes.
Meanwhile, remove rack from roasting pan and place pan on top of stove over medium heat. Cook until liquid is slightly reduced, about 5 minutes. Pour reduced liquid over chickens, or serve on the side.