Au blue is a French term for fish prepared instantly after it is killed. Most commonly used with trout, as in the dish truite au bleu.
In the typical preparation, the fish is is stunned with a blow, gutted, and soaked briefly in vinegar. The acid reacts with the fish's natural exterior mucus, causing it to turn blue. The fish is then poached in court-bouillon and served with a white sauce, butter, or Hollandaise Sauce, usually accompanied by boiled potatoes, parsley, and lemon weges. In some preparations, the vinegar step is omitted and the fish is poached in acidulated court-bouillon, which has a similar effect.
Name origins
Literally, "at the blue", from French. Introduced in Varenne's Le Cuisiner François, 1651.