Direction

1. Gather all ingredients. Line a 6-quart slow cooker with a disposable liner or generously coat with nonstick cooking spray.
2. Whisk together milk and sour cream dip in a bowl.
3. Melt butter in a small saucepan over medium heat. Whisk in flour and 1/2 teaspoon salt. Cook, stirring constantly, for 1 minute. Stir in milk mixture and bring to a boil, whisking constantly, over medium-high heat. Reduce heat to medium and simmer, whisking constantly, about 1 minute.
4. Remove pan from heat and stir in Cheddar cheese until melted, about 15 seconds.
5. In a small bowl mix together Parmesan cheese, garlic powder, remaining 1/4 teaspoon salt, and pepper.
6. Layer half of the potatoes in the bottom of the prepared slow cooker. Sprinkle with half of the Parmesan mixture. Top with onion and half of the cheese sauce. Layer remaining potato slices in the slow cooker and sprinkle with remaining Parmesan mixture. Pour remaining cheese sauce over the potatoes.
7. Cover opening of slow cooker completely with paper towels.
8. Cover with the lid and cook on High until potatoes are tender, 3 to 4 hours, replacing the paper towels once halfway through cooking. Serve topped with chives.