In a large pot, on medium heat, melt butter. Add celery, leeks, onion and garlic and sauté for 10 minutes until vegetables are translucent.
Add flour and stir for another 2 minutes. Add broth a simmer for 15-20 minutes.
Using a hand mixer, puree the soup until desired consistency. Stir in crème fraiche or sour cream and cook on low heat for another 2-5 minutes. Serve hot with garnishes of liking.