Heat oil in large saucepan; sauté onion, carrots, fennel, potato, turnip and ginger for 5 minutes then cover and cook on low heat for 15 minutes, stirring occasionally. Add water, salt & pepper and bring to a boil. Cover and simmer 20 minutes or until veggies are soft. Puree soup in blender until smooth. Add milk and gently reheat. In a small saucepan, melt butter; stir in flour and cook until thickened to make a roux, about 30 seconds. Add roux slowly to soup, stirring constantly. Cook until slightly thickened. Top each bowl with dill weed and crème fraiche, if desired.