Trim the green ends off the leeks. Cut the white ends lengthwise into quarters and thoroughly wash... leeks can be sandy. Then chop the leeks into small pieces. Place a large saucepot over medium heat and add the butter. Once the butter has melted add the leek and garlic. Saute for 3-5 minutes, then scoop over and set aside.
Add the mushrooms and thyme. Saute the mushrooms until the moisture has released and evaporated, 10 minutes. Then scoop 3/4 of the mushrooms out and set aside. Add the leeks back to the pot.
Pour in the stock, wine and sherry. Bring to a boil, then lower the heat. Simmer for 15 minutes. Puree the soup using an immersion blender. Then add the remaining mushrooms back to the pot. Stir in the cream and season with salt and pepper to taste.