Reserve the onions used to marinate the chicken and put in a small sauce pan.
Approximately 1 hour into the roasting of the chicken there should be a good amount of stock in the tin, pour the stock onto the onions and add about 350 ml of water. Whilst the chicken is still in the oven, bring the sauce to the boil and leave to simmer for 15 minutes.
Add the chopped coriander and leave to simmer and reduce for 30 minutes.
Take the saucepan off the hob and blend the sauce with a stick blender until smooth.
Taste and adjust the seasoning.