Wash the baby potatoes thoroughly. Leave the smaller ones whole and cut any very large potatoes in half. Slice the kielbasa into 1-inch pieces and roughly chop the onion
Place the prepared potatoes and kielbasa slices in the bottom of a large slow cooker. Sprinkle the chopped onion evenly over the top.
In a small bowl, whisk together the brown sugar, apple cider vinegar, Dijon mustard, and water until well combined.
Pour the sweet-and-sour mixture over the potatoes, kielbasa, and onion, distributing it as evenly as possible.
Cover and cook on LOW for 6–7 hours or on HIGH for 3–4 hours, until the potatoes are tender.
Gently stir everything to coat the potatoes and sausage with the flavorful sauce. Serve warm and enjoy!