Place the potatoes in a baking dish 24 hours ahead and bake at 350 degrees for an hour or until fork tender.
Slice the potatoes as thin as possible. Spray a casserole dish with cooking spray.
Place the potato slices on the bottom of the casserole dish.
Top with a layer of onions. Set aside.
Melt the butter in a medium saucepan. When it starts to sizzle, add the Carbquick a tablespoon at a time until it is slightly browned.
Begin adding milk a cup at a time while whisking to make sure there are no lumps. Whisk and stir until the mixture thickens.
Add a half cup of each type of cheese and 1/4 cup of the chives. Stir until the cheese is melted.
Once the cheese is melted, season the mixture with salt and pepper. Pour the sauce over the potatoes.
Top with the remaining cheese and sprinkle with some additional chives, black pepper or bacon bits.
Place the dish on a baking sheet and bake at 350 degrees until the top is golden.
Allow to cool 10-15 minutes before serving this amazing sidedish.