Put the onions in a large pan with the turkey neck (optional). Pour in stock and simmer for 1 hr. Strain to remove the vegetables and turkey, then return the liquid to the pan. For a textured gravy, remove the turkey and, using a stick blender, blitz the onion into the stock until smooth.

Blend the flour with the white wine or water, then blend this into the stock, stirring over the heat until thickened. Cool, then chill. Will keep for several days or freeze for one month. Thaw in the fridge a day ahead. Heat in a pan until piping hot, then add the juices from the turkey and season to taste.