Open the bag and dump the frozen meatballs directly into your slow cooker.
In a microwave-safe bowl, melt the butter. Add the flour, then microwave on high for 1 minute. Whisk well.
Microwave for 1 more minute, whisk again, then microwave for a final minute until bubbly to create a smooth roux.
Carefully whisk in the beef broth until completely smooth. Add the cherry preserves, Worcestershire sauce, Dijon mustard, and black pepper. Whisk until combined.
Pour the prepared sauce evenly over the frozen meatballs in the slow cooker.
Cover and cook on LOW for 6-8 hours, or until the meatballs are heated through and the sauce is deliciously thickened.
Just before serving, gently stir in the sour cream until fully incorporated for a rich, creamy finish. Keep warm until ready to serve.