Chop up your leek and soak in a bowl of cold water
While they are being soaked, heat up some oil in a small pan
Add the leek into the pan. Don't caramelize them, just tender them.
While the leeks are being tenderized, chop the potatoes up and boil them for a while. Keep some water over, you'll need it later.
When they have boiled, mash them, then add a generous amount of cream
As it's a bit thick, add the water you kept over from the boling potatoes and mix the leeks in.
Don't forget the salt and pepper!