In a food processor, chop mushrooms very fine.
Place in a soup kettle with broth and onions.
Simmer, covered, 25 to 30 minutes.
Meanwhile, melt butter in a 2-quart saucepan over medium heat.
When bubbly, stir in flour with a whisk to make a roux.
Cook and stir 3 minutes.
While roux cooks, bring milk just to a boil.
When roux is done, add milk all at once, stirring or whisking vigorously until smooth.
Cook and stir white sauce over medium heat 5 minutes, until thickened and smooth.
Stir in cream.
Add to broth mixture, stirring with a spoon until smooth.
Season to taste with salt and pepper.
Add sherry and heat through.