Rinse the mushrooms in a colander.
Melt butter in a large pot over medium heat. Add chopped onion and cook until softened.
Add mushrooms to the pot and cook until browned and softened.
If using, deglaze the pot with dry sherry.
Pour in chicken or vegetable broth and bring to a simmer.
Reduce heat and simmer for 30 minutes.
Stir in heavy cream and half-and-half. Heat through, but do not boil.
Season with cayenne pepper, white pepper, salt, and pepper to taste.
Serve hot, garnished with croutons and roasted mushrooms.