Ingredients :

• Firm white fish — Cod, pollock, ling, sea bass — whatever is fresh at your fishmonger that day. The important thing is the thickness: minimum 2 cm, otherwise the pieces will shrink to nothing during cooking. Avoid fish that are too thin like sole or plaice — they melt completely into the sauce and disappear.
• Leek — Use mainly the white and pale green parts, not the dark green top leaves which are too fibrous. Cut it into thin rounds so it melts quickly in the butter without remaining crunchy. One medium leek is enough — it’s not the star; it provides tenderness and a discreet sweetness.
• Fennel — This is what makes this recipe distinctly French. The anise flavor is very mild once sautéed in butter, nothing like raw fennel. If you can’t find any, substitute with celery, but you’ll lose something authentic. Half a bulb is plenty.
• Fresh dill — Not dried. Truly fresh. Dried dill is tasteless here. Add it off the heat at the last second before assembling to keep its bright green color and delicate fragrance. Heat kills it in minutes.