Heat olive oil and butter in a large pot over medium heat. Add sliced onion and cook for 15-20 minutes until caramelized.
Add mixed wild mushrooms and cook for an additional 5-7 minutes until tender.
Pour in vegetable broth and stir to combine all ingredients.
Add thyme leaves and bay leaf, season with salt and pepper, bring to a boil, then simmer for 20 minutes.
Blend the soup if a smoother texture is desired.
Stir in heavy cream if using, heat through for 5 minutes, remove bay leaf, and garnish with parsley before serving.