Cut off roots and trim dark-green top from leek. Discard any tough outer leaves. Thinly slice leek. Rinse leek thoroughly in bowl of cold water; swish to remove any sand. With hands, transfer leek ...

I choose fresh and firm fillets of blue grenadier, cod, or halibut ... think its taste is too dominant. The sauce will be excellent if you first sauté leeks and onions in butter.