Gather all ingredients.

Melt butter in a large soup pot over medium-high heat. Sauté mushrooms and 1 pinch salt in the melted butter until mushrooms release their juices, 5 to 10 minutes. Reduce heat to medium-low and continue to cook, stirring often, until juices evaporate and mushrooms are caramelized, 15 to 25 minutes. Set aside a few attractive mushroom slices for garnish later, if desired.

Add onion to the mushrooms; cook until onion is soft and translucent, about 5 minutes.

Stir flour into the mushroom mixture and cook, stirring often, to remove the raw flour taste, about 2 minutes.

Add thyme bundle and garlic cloves, then pour in 4 cups chicken broth and 1 cup water. Reduce heat to low and simmer for 1 hour. Remove and discard thyme bundle.

Purée soup with a blender in batches until smooth and thick.

Stir in cream. If too thick, add a little chicken broth or water. Season with salt and black pepper.

Ladle into bowls, and garnish with reserved mushroom slices and thyme leaves.