Pat the chicken dry; generously season the skin and inside the cavity with salt and pepper. Refrigerate, uncovered, at least 4 hours or overnight to dry out the skin. Let stand at room temperature 30 minutes before roasting.
Preheat the oven to 425 degrees F. Mash the butter with the chopped thyme and chopped rosemary in a small bowl. Gently slide your fingers under the chicken skin on the breasts and legs to loosen it. Scoop some of the herbed butter with a spoon; slide the spoon under the skin and hold the skin in place while you remove the spoon. Smooth the skin to distribute the butter. Repeat with the remaining butter.
Squeeze a lemon half into the cavity of each chicken, then stuff the lemon half inside. Stuff 1/2 onion, 3 garlic cloves, 1 thyme sprig and 1/2 rosemary sprig into each chicken cavity. Tie the legs together with twine. Set a rack in a large roasting pan; put the chicken on the rack and roast 25 minutes. Toss the scallions, tomatoes, olive oil and a large pinch of salt in a bowl, then add to the roasting pan. Remove the twine from the chicken and continue roasting until the skin is golden brown and a thermometer inserted into the thigh registers 170 degrees F, about 50 more minutes.
Transfer the chicken and vegetables to platters and let rest about 10 minutes before carving.