Line a 6-quart slow cooker with a disposable liner or coat with nonstick cooking spray.

Whisk together milk and sour cream dip in a bowl.

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 consistently over medium-high heat. Reduce heat to medium and simmer for about 1 minute.

Remove pan from heat and stir in Cheddar cheese until melted, about 15 seconds.

In a small bowl mix together Parmesan cheese, garlic powder, remaining 1/4 teaspoon salt, and pepper.

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.

Cover the opening of the slow cooker with paper towels and the lid.

Cook on High until potatoes are tender, 3 to 4 hours. Replace paper towels halfway through cooking.