Once you have roasted (and served if you wish) your chicken and it has cooled, carefully peel the skin off and put to one side.
Boil four cups worth of water and place in a small pan on a high heat. Boil the chicken in this water for about 10 minutes.
After ten minutes add wine, rosemary and crushed garlic.
Leave to boil for another 5 minutes.
Once you can smell the flavours really starting to come together, transfer into a skillet on a medium heat on a larger ring (the ring that you cook on should always correspond to the size of the pan you're using). Add the mushrooms.
Let the mix really boil down and reduce.
When reduced to your liking, sieve out the solids, putting the liquid back into the original pan.
Over a heat, add the double cream to the mixture and whisk with a sauce whisk. It should start to thicken.
If you've had problems getting it to thicken, feel free to "cheat" with a little cornflour.