Preheat oven to 325 °F.
Place chicken in a heavy-bottomed roasting pan and drizzle with oil. Sprinkle with salt and pepper.
Insert thyme sprigs into cavity of chicken, cover and roast for 20 minutes.
Uncover and continue roasting until an internal temperature (taken between thigh & breast) reads 180 °F, about 1 ½ hours.
While chicken is roasting, prepare glaze. Whisk honey and mustard. Purée onion in a food processor and stir into honey mustard.
During last 45 minutes of cooking, baste chicken with honey mustard, basting every 10 to 15 minutes.
Alternately, chicken can be taken off the bone, cut up and tossed with honey mustard glaze.