1. Preheat oven to 375 degrees. Cook potatoes in simmering salted water until just tender, 2 to 3 minutes. Drain.

2. Place chicken in a roasting pan, breast side up. Scatter potatoes, garlic, 6 thyme stems, and 1/2 of the lemon slices around chicken. Dot potatoes with 1 tablespoon butter divided into small pieces.

3. Stuff chicken cavity with 1 lemon half and 3 thyme stems. Truss chicken. Rub remaining 3 tablespoons butter on skin.

4. With your fingers, loosen skin from chicken breasts. Push remaining lemon slices and thyme stems under skin in a single layer. Squeeze juice from remaining lemon half over chicken and potatoes, and season with salt and pepper.

5. Roast, turning potatoes 2 or 3 times, until a meat thermometer inserted into thickest part of chicken thigh registers 165 degrees, about 1 1/2 hours. Let stand for 15 minutes. Garnish with thyme sprigs.