1Mix crème fraîche and 1/4 teaspoon of the tarragon in small bowl. Cover. Refrigerate until ready to serve.
2Melt butter in large saucepan on medium heat. Add leeks and shallot; cook 10 minutes or until softened, stirring occasionally. Add potatoes, chicken stock and remaining 1 tablespoon tarragon. Bring to boil on high heat. Reduce heat to low; cover and simmer 20 minutes or until potatoes are very tender.
3With center part of cover removed to let steam escape, puree soup in batches in blender on high speed until smooth. Return pureed mixture to saucepan. Add cream, sea salt and white pepper. Cook on low heat until heated through. Ladle into soup bowls. Top each serving with a dollop of crème fraîche mixture.