Sweating onions is a cooking term that refers to softening the onions first and cooking just long enough to enhance the natural sweetness they contain but not so much as to let them turn brown. Usually either butter or vegetable oil is used to perform this cooking technique. To help seal in the sweet juices and keep the onions moist, a cartouche (a layer of greaseproof paper) is usually placed over the onions during the sweating process. Once sweated, the onions can be used in the recipe as directed.