Preheat the oven to 425 degrees.
Remove the chicken giblets from each of the chickens. Rinse the chickens inside and out and remove any excess fat and pinfeathers. Pat the outsides dry with paper towels. Place the chickens in a large roasting pan. Liberally salt and pepper the cavities of the chickens. Stuff the cavity of each chicken with half of the thyme, half of the lemons, and half of the garlic. Brush the outside of the chickens with the butter and sprinkle them liberally with salt and pepper. Tie the legs together with kitchen string and tuck the wing tips under the body of each chicken. Scatter the onion slices around the chickens, drizzle them with olive oil, toss the onions with your hands, and distribute them around the chickens. (You want the chickens and onions to fit snugly into the pan or the onions will burn.
Roast the chickens for 1¼ to 1½ hours, or until an instant-read thermometer reads 155 to 160 degrees and the juices run clear when you cut between a leg and thigh. Cover with aluminum foil and allow to rest for 15 minutes. Slice the chickens onto a platter and serve immediately with the onions and pan juices.
Copyright 1999, The Barefoot Contessa Cookbook, Clarkson Potter/Publishers, All Rights Reserved/updated