Melt butter in a pot set over medium heat. Mix in celery and onion and cook until tender. Mix in thyme, potatoes and chicken stock. Simmer soup 20-25 minutes, or until potatoes are tender. Puree mixture in a food processor or blender. Return to soup pot. Add milk and season with salt and pepper. Bring up to a simmer and then pour into soup bowls. Garnish with celery leaves and serve.
Note: Celery is an excellent source of potassium and also contains vitamin C, folic acid and vitamin B6.