What is the term for the French technique of cooking meat in parchment paper?

The term "En Papillote" refers to a classic French cooking technique that involves placing food, often meat or fish, in a folded pouch or packet made from parchment paper. This method allows the ingredients to steam in their own juices as they cook, preserving moisture and enhancing flavors. When the pouch is closed, the steam generated inside helps to infuse the dish with aromas, creating a tender and flavorful result.

In contrast, the other techniques listed have different applications. "Au Gratin" refers to a cooking method that involves browning a dish, usually topped with breadcrumbs or cheese, under a broiler. "Braising" is a technique that combines both dry and moist heat, typically involving cooking meat slowly in a covered pot with a small amount of liquid. "Sous Vide" is a method that involves vacuum-sealing food in a bag and cooking it in a water bath at a precise, low temperature for an extended period.

The choice of "En Papillote" is aptly aligned with the description of cooking meat (and other proteins) in parchment, making it the correct answer for this question.