1. Heat oven to 400 degrees F with rack in the lower third. Sprinkle the inside of each chicken with 1/4 tsp salt, then fill with some onion, lemon, celery and herbs. Place extra vegetables and lemon in the bottom of a roasting pan. Tie chicken legs together with kitchen string, then place chickens on top of vegetables in pan and pat dry with paper towels. Rub each with butter and sprinkle with remaining salt.
2. Roast until golden, tenting with foil if parts become too brown. Cook until a meat thermometer reads 170 degrees F when inserted under the thigh and juices run clear (dark meat may be slightly pink), about 1 hr 20 min. Remove from oven and let rest 10 min. Serve hot or let cool about 45 min; wrap leftovers loosely and store in fridge up to 3 days.
3. If desired, remove vegetables from roasting pan and transfer juices and any browned bits to a saucepan. Place over heat, add broth and bring to a boil; reduce liquid by half (about 10 min), pour into serving dish and serve alongside chicken. Makes 2 whole chickens.