Slice the mushrooms thinly. Cut the onion in half, then slice it thinly. Mince the garlic.
Melt the butter in a large soup pot. Add the mushrooms, onion, and garlic. Sauté until tender, about 10 minutes.
During this time, chop the parsley and remove the thyme leaves from the stems. When the mushrooms are tender, add the herbs and tomato paste. Simmer about 1 minute.
Add the white wine, broth, parmesan cheese, salt and pepper.
Bring to a boil, then reduce to a simmer. Simmer 1 hour.
Serve with more parmesan as garnish, plus croutons if you like.