Once meat is cooked, remove from slow cooker and let sit for 15-30 minutes. During this time, bring the liquid left behind from cooking to a boil (either in the slow cooker, depending on the model you have, or transfer to stove-top) and reduce liquid by half. Once the liquid is reduced by at least half, and coats the back of a spoon, remove from heat. Immediately whisk in 2 Tablespoons cold unsalted butter. You may remove the onion, or leave it.