Heat olive oil in a saucepan over medium-low heat. Add onions and cook until soft, 5-7 minutes. Add garlic and mushrooms and cook until mushrooms are completely soft and browned, at least 20 minutes.

Add chestnuts and wine and cook for 2 minutes. Stir in onion soup mix. Add chicken stock and bring to a boil. Lower heat and simmer for 20 minutes.

Using an immersion blend, blend the soup a bit but not too much. The soup will be a more broth-like consistency at this point. Let it continue to cook if you prefer it thick and creamy.