Pick a handful of potato slices and start organizing them into a neat stack in the casserole dish with their edges aligned vertically. Continue placing potatoes in the baking dish, working around the perimeter and into the center until very tightly packed. Do the same in the other baking dish.
Add leftover milk and cheese liquid to the baking dishes.
Cover baking dishes with foil and bake 30 minutes, then remove the foil and bake 15 minutes more until the top is golden brown.