The day before you want to serve this dish, cut the leeks from top to root and wash thoroughly under cold running water. Thinly slice them on the diagonal and put into plastic bags and chill until ready to cook.
To serve, put a large pan over a medium heat, add the butter and let it melt over the base of the pan. Add the leeks and plenty of seasoning and stir to coat in the butter. Turn the heat down to low, cover the pan and cook the leeks gently for about 15 minutes, stirring half way through until they are tender. Serve with extra butter and a sprinkling of thyme leaves.