On this page of our Assassin's Creed Valhalla guide, you will find a walkthrough of the Ledecestrescire Sauce side mission.
Award for completion: 1200 XP.
On the place marked in the picture above, you will encounter a man who is worried that he has lost his talent for cooking. To finish his wonderful sauce he will need the last ingredient - eel.
Go to a nearby pier and locate the fish using Odin's Sight. You can just shoot them using a bow.
When you manage to catch an eel, go back to the cook and add the ingredient to the boiling sauce to complete the task. You can fulfill the cook's last request and take his "failure" to the basement. You will find the entrance to the basement on the right side of the cottage.
It is worth taking this step, as you will be able to pick up some items from the chest that was cleverly hidden there.