Mash the garlic into the butter and then spread it over the chicken.
Put all the vegetables into the roasting bag on a baking or roasting tray. Put the chicken in the bag on top of the vegetables. Pour the stock into the bag and fasten the bag using one of the ties that comes with it.
Put it in the oven for 2 hours. If your chicken is much larger or smaller you may need to adjust the cooking time. As you can't check very easily if the chicken is cooked then it's best to overestimate the time as it will still stay moist in the bag.
Let the chicken rest for 20 minutes after it comes out of the oven before serving.
Nutritional information is approximate and a guideline only.