Au Torchon is a French phrase used to describe a cooking technique.
A “torchon” is a cloth, such as a dish towel, that a food is wrapped in for cooking.
A cheesecloth can be used, or a pudding cloth (which has a tighter weave), or a clean kitchen towel. You wrap the food item in the cloth, and tie it securely with kitchen string.
The food item might then be marinated, or poached, or both.
Handling a food item in this manner both shapes the food, and when used with items such as foie gras, stops all the fat leeching out during poaching (which would defeat the purpose of having foie gras in the first place.)
For chou-fleur (cauliflower) au torchon, you use a cloth and string to make a very tight bundle of buttered cabbage leaves and cauliflower pieces, which you then simmer in water for a few hours. The cloth keeps the ingredients together.
For jambon (ham) au torchon, you wrap tightly and secure a brined pork loin in a muslin cloth, and poach it for about an hour. The cloth shapes the meat.
For gigot cuit au torchon (top half of rear Lamb Leg cooked in a towel), the meat is browned, then secured in a towel, and braised for 7 hours. The cloth ensures that the meat stays together and doesn’t fall apart in the casserole dish.
Literature & Lore
Awareness of the “Au Torchon” technique seems to have been popularized in recent times by a recipe for “Foie Gras Au Torchon” by Thomas Keller in his 1999 book, “The French Laundry Cookbook .” The recipe calls for the wrapped foie gras to be poached for 90 seconds.
In Belgium, “torchon” is more used to refer to a rag to clean floors with, than something you’d imagine wrapping around your dinner.