In a large dutch oven, heat the butter over low heat with a pinch of salt. Add the onions and celery and cook for several minutes until translucent. Add garlic and mushrooms and continue cooking. The mushrooms will release a lot of liquid. Continue cooking over low heat, stirring periodically, until very little liquid remains in the pan, approximately 5 minutes.

Turn the heat up to medium low and allow the mushrooms to begin caramelizing on the bottom of the pan for flavor. Stir periodically and scrape up the brown bits from the bottom of the pan, incorporating them back into the vegetables. This can be done briefly or for several minutes. The longer they caramelize, the more flavorful the soup will be.

Pour the brandy into the pan and use the liquid to help scrape up any remaining brown bits. Allow the brandy to cook down to almost nothing and then add the stock and thyme leaves. Bring the soup to a boil and then cover and reduce the heat to low. Allow the soup to simmer for one hour.

Ladle the soup into a blender and puree until silky smooth. Pour the soup back into the pot and add the cream, salt and pepper. If a thinner consistency is desired, add more cream.

Thinly slice the pale green section of the spring onions to use as garnish.