Melt butter on low. Add onions to butter and sauté at med-high heat until translucent.
Once onions are translucent, add garlic and mushrooms, and light salt and pepper. Maintain med-high heat, and let mixture
Cook down. Add salt and pepper. as needed. (Mushrooms will release lots of liquid.)
Put cream into a second pot, and heat. Add chicken stock and herbs to cream, then bring to a boil.
Add Marsala to mushrooms. Bring marsala and mushrooms to a boil, then reduce to half of original volume.
Once reduced, add chicken stock and cream to mushrooms. Bring back to a boil. Make sure anything on sides of pot goes back into soup.
Reduce to desired thickness (approx. 5 min). Salt and pepper to taste. Add splash of vinegar.
Garnish with parmesan