Preparation Melt butter in heavy large pot over medium heat. Add leeks, onion, garlic, and 2 tablespoons water. Cook until leeks are just golden, about 10 minutes. Add potatoes and broth; bring to boil. Reduce heat to low and simmer until potatoes are tender, about 10 minutes. Mix in tarragon. (Can be made 1 day ahead. Cool slightly. Cover and refrigerate. Rewarm before continuing.) Stir in cream and yogurt. Season soup to taste with salt and pepper.
Preparation Melt butter in heavy large pot over medium heat. Add leeks, onion, garlic, and 2 tablespoons water. Cook until leeks are just golden, about 10 minutes. Add potatoes and broth; bring to boil. Reduce heat to low and simmer until potatoes are tender, about 10 minutes. Mix in tarragon. (Can be made 1 day ahead. Cool slightly. Cover and refrigerate. Rewarm before continuing.) Stir in cream and yogurt. Season soup to taste with salt and pepper.