Serve chicken with rustic bread spread with the cloves of roasted garlic.
Heat oven to 425 degrees. Rinse chicken; pat dry. Season with salt and pepper, including cavity. Pick thyme leaves from a quarter of the sprigs; set aside. Separate garlic cloves from half the heads. Stuff chicken with a handful of garlic cloves, a few thyme sprigs, and a few lemon quarters. Truss chicken.
Strew roasting pan with remaining cloves, lemons, and thyme. Set chicken over bed; sprinkle with reserved thyme. Roast until chicken is golden and, when thigh is pricked, juices run clean, about 1 hour and 10 minutes.
Scatter parsley on platter; place chicken on top. Serve with garlic, lemon, and juices that have collected in bottom of pan.