Preheat oven to 475 degrees. Place chicken in a roasting pan and season all over with salt and pepper. Gently loosen skin and rub chicken breasts with thyme leaves, salt, and pepper. Stuff cavity with 5 thyme sprigs, garlic, and lemon. Using kitchen twine, securely tie legs together; tuck wing tips underneath.
Roast chicken for 15 minutes. Remove from oven, baste with pan juices; add shallots, pears, and remaining thyme sprigs to pan. Return to oven and continue to roast, until thermometer inserted in the thick part of the thigh (avoiding the bone) reaches 165 degrees, 45 minutes to 1 hour, basting and tossing pears and shallots once. Transfer to a serving platter and let rest 10 minutes before serving.
To reduce the fat in this dish, remove the chicken's skin before serving.