A cartouche is a piece of parchment paper cut and tinted over a braised food item while it cooks. The goal of a cartouche is to prevent the item from drying out during the braising process.
Braising is a moist heat cooking method in which a large, typically multi portion serving of meat is semi submerged in cooking liquid and simmered until tender. Tough cuts of meat are typically ideal for braising such as chicken legs, veal or lamb shanks, short ribs, or brisket, and pork shoulder.
Once the food item has been optionally seared and placed in liquid, a piece of parchment is cut to cover the meat with the goal of retaining moisture around the exposed surface of meat. This is a cartouche. When cooking in a hotel pan, a half sheet of parchment is typically laid over the food and then covered with a lid. When using a smaller pot or rondeau, the parchment can be folded and cut into a circle with a tiny vent hole cut in the middle to prevent full on steaming.