Rub some olive oil all over the chicken and sprinkle it with salt and pepper. Into the cavity place the onion, celery, orange, garlic and thyme. Place on an oiled and suitably sized roasting dish in the middle of the oven for about 30 minutes.
Take the chicken out. Whatever juices have been released by the chicken, tip the dish, collect them in a spoon and pour over the chicken. Sprinkle the onion with a little salt, pepper and thyme leaves, Surround the chicken with the chopped onion. Turn the oven down to 375 F and put the dish back in the oven. During this time take the chicken out once or twice and again spoon the roasting juices over the bird. Also move the onions around at these times so they roast evenly.
When the chicken is done take it out and leave it, in the pan, to rest for 15 minutes. This is very important as it will keep the meat moist. Then remove it to a serving dish. Now scrape all the onions, juices, fat - basically anything left in the pan into a large microwaveable bowl. Add in the orange, mustard and a sprinkle of salt. With a hand blender blend to as smooth a consistency as you like. Taste and add a little more salt if necessary. If it's too thick add a little more hot water. If it's not as piping hot as you'd like, microwave it for 30 sec to 1 minute depending.