Put oven rack in upper third of oven and preheat oven to 500°F.
Toss chicken with oil, thyme sprigs, garlic, salt, and pepper in a large bowl, then transfer to a large (17- by 12-inch) shallow heavy baking pan (1 inch deep).
Bake chicken 10 minutes, then add lemon slices to pan (or slip under skin for added flavour). Continue to bake until chicken is golden and cooked through, 15 to 20 minutes more. Serve chicken with lemon slices.