Melt butter in a large soup pot over medium-high heat. Sauté mushrooms and pinch of salt in the melted butter for 5-10 minutes or until mushrooms release their juices. 

Reduce heat to medium-low and continue to cook until mushrooms are carmelized, about 15-25 minutes. Stir often. 

Add onion to pot and cook until onion is soft and translucent, about 5 minutes. 

Stir flour into the pot and cook for about 2 minutes, stirring often. 

Add bundle thyme and garlic cloves, then pour in the chicken broth and water. Reduce heat to low and simmer for 1 hour before removing the bundle of thyme. 

Using a blender, purée soup until smooth and thick. You will likely need to purée in batches. 

Stir in heavy whipping cream. Add a little broth or water if your soup is too thick. 

Season with salt and pepper and serve hot.