Heat olive oil and butter in a large pot over medium heat. Add chopped onions and garlic, cooking until softened.
Add mushrooms to the pot and cook until browned. If using white wine, add it now and let it reduce for a minute.
Pour in vegetable broth and bring to a boil. Lower heat and simmer for 15-20 minutes.
Stir in cream and season with salt, pepper, and fresh thyme. Let it simmer for another 5 minutes.
Meanwhile, heat oil in a pan and fry the leeks until golden and crispy, about 3-4 minutes.
Blend the soup until smooth, or leave it chunky for texture.
Serve the soup in bowls, topped with crispy leeks.