Stir flour into mushroom mixture and cook, stirring often, for 2 minutes to remove raw flour taste. Add dried thyme and garlic, then add chicken stock and water and bring to a simmer. Cook on low for 45 minutes.
Transfer soup to a blender and puree in small batches or use a hand blender to puree to desired consistency, leaving some of the mushrooms intact.
Return soup to pot (if using blender) and stir in cream. Season with salt and black pepper.