Heat the olive oil in a large saucepan over medium heat. Add leeks and garlic; sauté for 5 minutes or until soft.
Add mushrooms, thyme, tarragon, salt and pepper; sauté for 5 minutes. Stir in wine and broth; cook for 20 minutes.
Transfer mixture to a blender or food processor; puree until smooth. Return to saucepan and heat through. Stir in yogurt; cook for 3 to 4 minutes or until warm.
Serve with goat cheese and baguette.