Heat a large saucepan over medium heat. Add the butter and oil and then the garlic, onions, celery and celery root. Season with salt and pepper and cook while stirring for 15 minutes. Stir in chicken broth and thyme, bring to a boil and then turn down to simmer for 25 more minutes, until vegetables are very tender. Season again with salt and pepper. Turn off the heat and puree with an immersion blender. Put the soup back over low heat and stir in the cream. Serve garnished with celery leaves.