Melt the butter in a heavy-bottomed saucepan. When
it foams, add the potatoes, onions and leeks and
toss them in the butter until well coated. Season
with salt and pepper.
Cover and sweat on a gentle
heat for 10 minutes or until the vegetables are
soft.
Add the stock and boil until the vegetables
are just colored.
Add the chopped nettle leaves
and simmer for a few minutes, being careful not to
overcook. Add the cream or milk and liquidize. Serve
hot.