Bring the chicken out of the refrigerator about 30 minutes before you start prepping so that it is at room temperature and pat it dry.
Preheat oven to 450°F (220°C) conventional oven or 430°F (200°C) fan forced.
Prepare the seasonings: Peel the onion and cut it into quarters. Peel the garlic cloves. Wash the thyme leaves and mix some with the olive oil.
Prepare the stuffing: Rinse the whole chestnuts and place in a bowl with the onion, garlic, butter, oil, salt and pepper. Give it a little stir.
Rub butter, oil, thyme, salt and pepper all over the inside and outside of the chicken. You can also add some of this mixture under the breast skin. Just lift the skin that is above the breast gently with a spoon or your fingers and add a bit. It will make your breast meat juicier and more flavorful. But if you don't have time, it's fine. The roast chicken will still taste amazing. Also, lightly butter the roasting pan.
Put the stuffing into the cavity of the chicken.
Place the chicken breast-side up directly in the roasting dish and put the chicken in the preheated oven.
Peel the potatoes and cut them in half or in quarters if they are larger. If they are smaller, you can just leave them whole.
Drizzle a bit of olive oil over the remaining chestnuts and give them a little stir to coat them evenly.
After the chicken has roasted in the oven for 15 minutes, lower the oven temperature to 350°F (180°C) conventional oven or 325°F (170°C) fan forced scatter the potatoes and mushrooms around the chicken and drizzle a little olive oil over it all.
Every 30 minutes or so, baste the chicken and move the vegetables around a bit, so that they coat with the roasting liquid of the chicken and roast evenly. You can also place the chicken on top of the vegetables and have them roast underneath. Make sure you coat the chestnuts with the chicken juices and oil, otherwise they can dry out. (Note: if your chestnuts seem a bit delicate, you can just add the remaining chestnuts about 15 minutes before the chicken is done - see note below).
Rough general rule: Roast chicken at 450°F (220°C) in a conventional oven or 430°F (200°C)°C in a fan-forced oven for 15 minutes. Then reduce the oven temperature to 350°F (180°C) and roast for 20 more minutes for every one pound/500grams. See notes for more details.
A 4-pound (1.8 kg) chicken will be done in about 1 hour 35 minutes (total time, including the first 15 minutes at a higher temp). To check to see if the chicken is done, place a meat thermometer in the thickest part of the chicken and the temperature should measure 165°F (74°C). If you pierce the chicken in the thickest part, the juices should run clear and not pink.
You should also check the temperature of the stuffing, which should also reach a temperature of 165°F (74°C).
Rest the chicken, without removing the stuffing, for 20 minutes after you take it out of the oven.