Lightly spray the inside of your slow cooker with nonstick cooking spray.
Add the sliced kielbasa to the slow cooker.
In a small bowl, stir together the BBQ sauce, brown sugar, and Dijon mustard (if using).
Pour the sauce over the kielbasa and stir until everything is evenly coated.
Cover and cook on LOW for 3–4 hours or on HIGH for 1½–2 hours, stirring once if you think about it.
Serve warm directly from the slow cooker.
Notes
This recipe can be served as an appetizer or main dish.Dijon mustard is optional but adds balance to the sweetness.Cooking times may vary slightly depending on slow cooker size and heat.Leftovers store well in an airtight container in the refrigerator for up to 4 days.