Eel Sauce, Anago no tsume, for the Pro

Eel sauce is a by-product of a sushi restaurant’s daily preparation of conger eel. Every day, chefs fillet conger eels, anago, and cook them in a broth made from the eel bones and head seasoned with sake (rice wine), shoyu (soy sauce), and mirin (sweet cooking wine). When the conger eel is cooked until tender, the cooking broth is saved and recycled to cook the next day’s batch of conger eel, and then again the third day’s batch. By this time, the cooking broth i