Place onion through black pepper in your slow cooker, mix with a spoon. Place roast in slow cooker, smear the top of the roast with mixture. Set slow cooker for low for 8 hours or high for 4 hours.
Thirty minutes prior to serving, skim off any excess fat that has risen to the top and remove roast. Shred roast using two large forks. Return meat to the slow cooker and add unsalted butter and kale. If your sauce requires additional thickening, you can combine 2 tablespoons corn starch in 2 tablespoons of water, whisking out any lumps, and then stir into sauce.
Prepare desired pasta. Top pasta with Slow Cooker Ragu and enjoy!
Buying vegetables pre-chopped is a huge time saver.