Preheat the oven to 350 degrees F and lightly mist a 9x13-inch baking dish* with non-stick spray.
Slice the potatoes into 2mm rounds using a mandoline slicer or sharp knife, and arrange them in the baking dish in an even layer .
Combine the cream and milk and pour about 2 cups of the liquid over the potatoes, submerging them completely.
Melt the butter in a medium pot and saute the diced onion until translucent.
Add the remaining 1 cup of cream/milk mixture, cream cheese, garlic powder, salt, and pepper, and simmer gently, stirring, until the cream cheese has melted and the mixture is smooth,
Stir in about one cup of the shredded cheese until smooth and melted.
Pour the mixture over the potatoes and top with the remaining 1 cup of shredded cheese.
Cover the pan with foil and bake the scalloped potatoes for 30 minutes, then remove the foil and bake until the sauce is bubbling, the cheese is browning, and the potatoes are tender (approx. 30 minutes).
Let stand for 10 minutes before garnishing with fresh chives and serving.
Video
Notes
*This recipe can be halved and baked in an 8x8 square baking dish.