In the recipe for Spaghetti with Creamy Braised Garlic and Leeks, a piece of parchment is placed between the braising vegetables and the lid. Why? As the food braises, vapors rise, hit the pan’s lid, and drip back down. The farther they travel, the more they dilute. The parchment reduces the headroom in the pan, so the vapors travel less and become more concentrated, giving you a richer sauce.