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