Cartouche [kär too sh] n. Circle of greaseproof parchment used to cover a dish while poaching or simmering.
Poaching is a quick way to cook certain cuts of meat - especially fish and poultry. It usually involves cooking the food in a small amount of sauce or liquid, often covered by a cartouche. The cartouche serves two purposes: it keeps the small amount of liquid in a poached dish from evaporating too quickly, and it prevents that unpleasant skin from forming on the top of a sauce.
Here's how to make a cartouche, and some tips for its use in poaching. (Image credit: Screenshot from Kitchen Pirate video)