Over medium heat, in a large skillet, heat butter and oil together. Add dry chopped leeks, salt lightly and cook until just tender, 4-5 minutes. Do not brown.
Add the splash of wine and cook until almost gone, 3-4 minutes.
Stir in the potatoes, then add the stock and herbs, salt and freshly ground pepper, white or black.
Simmer the soup for 25-30 minutes or until the potatoes are easily pierced with a paring knife. Take the pan off the heat and remove the bouquet garni.
Carefully purée the hot soup with an immersion blender or in batches in a blender until smooth. Stir in the cream and gently reheat. Do not boil.
Serve hot or cold with a swirl of cream and a raw garnish of very finely sliced leek, slightly fluffed to separate the rounds, white part only.