Pat the chickens dry inside and out. Generously season the skin and cavity of each with salt and pepper. Refrigerate, uncovered, at least 4 hours and up to overnight to dry out the skin.
Let the chickens stand at room temperature for 30 minutes before roasting. Preheat the oven to 375 degrees F.
Mash the butter with the chopped thyme and rosemary in a small bowl. Gently slide your fingers under the skin on the breasts and legs of each chicken to loosen . Scoop some of the herbed butter with a spoon, slide the spoon under the skin on one chicken and hold the skin in place while you remove the spoon. Smooth the skin to distribute the butter; repeat with more butter if needed. Repeat with the second chicken and remaining herbed butter.
Squeeze the juice of a lemon half into the cavity of each chicken, then stuff the lemon half inside. Stuff 3 cloves garlic, 1/2 onion, 1 thyme sprig and 1/2 rosemary sprig into each chicken cavity. Tie the legs together with twine. Put the chickens on a baking sheet next to each other.
Toss the parsnip, potato, squash, olive oil and a large pinch of salt in a bowl, then add the vegetables to the baking sheet, scattering them around the chickens. Roast until the chickens are golden brown and a instant-read thermometer registers 165 degrees F, about 1 hour. Transfer the chicken and vegetables to platters and let rest about 10 minutes before carving.
Copyright 2017 Television Food Network, G.P. All rights reserved.