Like John - Cream of mushroom soup.
That's what I do when I have around 500g of mushrooms left, especially if they're beginning to look a bit soft, but can't give recipe as it's copyright in an old cookery book that belonged to my partner's mother.
Basically involves mushrooms, onions, small amount of butter / marg / oil & flour, stock, milk, with optional extras of egg and / or cream. A mixer that can be used in saucepan is very helpful to get texture just right.