Prepare your slow cooker by lightly greasing it with cooking spray or lining it with a slow cooker liner for easy cleanup.
Place the kielbasa slices evenly in the bottom of the slow cooker.
In a medium bowl, combine chopped apples, brown sugar, apple cider, Dijon mustard, and garlic powder. Season with salt and pepper and mix until fully combined.
Pour the apple mixture over the kielbasa, making sure all pieces are well-coated.
Cover and cook on low for 3–4 hours or high for 1–2 hours, until the kielbasa is heated through and the flavors have blended.
Gently stir before serving to coat the kielbasa in the sauce again.
Garnish with chopped parsley, if desired.
