Creamed turkey and mushroom soup

Put the turkey carcass, with any remaining turkey meat on it, into a large pan. Add the gravy, onion and herbs and pour in 1 litre (1¾ pints) water. Season well. Put the lid on the pan, bring to the boil and simmer for 30 mins. Lift out the turkey carcass, pulling off any turkey meat left on there, if you like. Add the mushrooms and simmer for 15 mins. Use a stick blender to half-blend the soup in the pan, or blend about half the soup in a food processor and then pour it back into the pan. Stir in the milk and warm through to serve. Add slivers of cooked turkey, if you like.