Lay the chicken on a cutting board breast-side down, with the legs closest to you. Use kitchen shears to cut out the backbone. Open the chicken like a book to reveal the ribs and sternum. Use a chef's knife to score the sternum vertically down the center, then use your hands and body weight to crack the sternum and flatten the breasts and rib cage. Season the chicken all over with 2 teaspoons salt and a generous amount of pepper.
Lay the thyme sprigs in the center of a rimmed baking sheet and place the chicken on top, skin-side up. Roast the chicken until the skin is golden brown and an instant-read thermometer inserted into the thickest part of the thigh registers 165 degrees F, about 40 minutes.