If you are using raw chestnuts, boil them and leave to cool before skinning.
Cook the diced onion in a pan, coated with a few squirts of Frylight, over a low heat for 15 minutes or so. Once softened add the garlic and sage and stir for a minute.
Pour in the freshly made stock and add the chestnuts. Bring to the boil and then simmer for 15 minutes.
Blend, check and adjust seasoning finally adding the creme fraiche.