(Recipe #17, pages 129 – 130)
Like all fish to be cooked au bleu, trout are not scaled but only gutted; it is best to lay them on a moistened cutting board and handle them as little as possible, so as not to rub the slime responsible for the blue color off the scales. Rinse the trout and lay them on a flat platter, pour hot vinegar over them, and let them stand covered for ½ an hour. This will turn them blue, but they still must not be touched.
Boil heavily salted water; when it comes to a boil, slide the trout with the vinegar into the water and let the kettle stand covered on the stove for 6 minutes, without letting the fish boil. Serve hot with melted butter and (optionally) finely chopped parsley or cold with good quality olive oil, vinegar, and pepper. For all fish preparations, the butter must not be heated to the foaming point as it melts, because if it that hot it loses much of its quality.
Trout are among the fish have greatest need of salt.
Note: In most cases, trout are probably cooked without first being turned blue; they are simply boiled in water as described.
Translated by David Green.