Chop garlic, then mash with salt to make a paste. Transfer to small bowl. Add oil, tarragon, basil, thyme, marjoram, oregano and pepper, and mix. Slip your fingers under the skin of each breast to loosen; rub as much herb paste as you can under skins. Rub remaining paste in body cavities, then place a lemon half in each cavity. If desired, tie chicken legs together with kitchen twine.
Place chickens, breast side up, on rack in large roasting pan. Roast 30 minutes, basting twice. Scatter onions, carrots and celery in pan. Roast about 20 to 30 minutes longer, basting once or twice, or until a meat thermometer inserted in thickest part of thigh, without touching a bone, reads 180°.
Transfer chickens to a platter. Pour pan juices into a small glass measuring cup. (Discard vegetables in roasting pan.) Let juices stand 5 minutes, then skim off any fat from surface. Place pan over high heat on stove. When pan is sizzling, pour in degreased juices and broth. Bring to a boil, stirring up browned bits on bottom of pan with a wooden spoon. Strain into a sauceboat. Carve chicken and serve with pan juices.
Adapted from Back to the Table by Art Smith