Truite au bleu

A truite au bleu, or trout au bleu, is a preparation of wild trout which is killed immediately prior to being poached in a vinegar court-bouillon. The fish takes on a blue color as the vinegar reacts with the natural slime on the surface of the fish's skin. Any delay in the preparation of the dish after killing the fish will allow the protective layer to run off, so au bleu can only be made with the absolutely freshest fish.