Melt the butter in a medium saucepan. Add the onion and leek and cook until softened - about 10-15 minutes on a low heat.
Add a splash of white wine, the potatoes, stock and seasoning and simmer until the potatoes are tender.
Liquidise using a hand-held blender, or transfer to a liquidiser. Pass through a sieve before adding the milk and cream.
Chill in the fridge until ready to serve.
Sprinkle with chives and parsley just before serving.