In a 4-quart saucepan cook in butter turmeric and finely chop shallots over moderately low heat, stirring, until softened. Add wine and stock and simmer mixture until reduced by half, about 25 minutes.
Add creme fraiche and heavy cream and simmer, stirring occasionally, 15 minutes.
Pour soup through a fine sieve into a heatproof bowl. Stir in white-pepper, vinegar and salt to taste. In another bowl whisk remaining 1/2 cup heavy cream until thickened and stir into soup until incorporated.
Serve with wedge of herbed flatbread.