Heat the butter in a large pan over medium heat. Add onion and thyme and cook 6-7 mins until translucent
Add in the mushrooms and cook, stirring frequently, for 5 mins or until they release some of their juices and are softened. Reserve a few pieces of sliced mushrooms for garnish. Add garlic and cook for 30 more seconds
Pour in the vegetable broth and water. Stir in the nutmeg, salt and pepper
Put the lid on, bring to the boil, turn the heat down to medium-low and simmer for 15 mins. Be sure to keep an eye on it and stir half way through
Off the heat, carefully puree the soup using a blender or food processor until smooth.
Ladle soup into bowls and garnish with fresh thyme and croutons. Serve while hot
Soup can be stored in an air-tight container in the fridge or freezer to be heated and enjoyed at another time