Step 1

Melt butter in a large soup pot over medium-high heat; cook mushrooms in butter with 1 pinch salt until the mushrooms give off their juices; reduce heat to low. Continue to cook, stirring often, until juices evaporate and the mushrooms are golden brown, about 15 minutes. Set aside a few attractive mushroom slices for garnish later, if desired. Mix onion into mushrooms and cook until onion is soft and translucent, about 5 more minutes.

Step 2

Stir flour into mushroom mixture and cook, stirring often, for 2 minutes to remove raw flour taste. Tie thyme sprigs into a small bundle with kitchen twine and add to mushroom mixture; add garlic cloves. Pour chicken stock and water into mushroom mixture. Bring to a simmer and cook for 1 hour. Remove thyme bundle.

Step 3

Transfer soup to a blender in small batches and puree on high speed until smooth and thick.

Step 4

Return soup to pot and stir in cream. Season with salt and black pepper and serve in bowls, garnished with reserved mushroom slices and a few thyme leaves.