Preheat the oven to 400 degrees F. Spray a 9x13 inch baking dish with nonstick spray.
Peel and slice your potatoes. I highly suggest (carefully!) using a mandolin to do so, in order to achieve potato slices that are the exact same width. I like to go with potato slices about 1/8 inch thick – or even a little less! Place the potatoes in a large bowl.
Heat the cream, milk, salt, pepper and nutmeg together in a saucepan over medium heat. Bring the mixture to a boil then remove it from the heat and let it sit for 5 minutes.
In another bowl, stir together the cheeses, breadcrumbs, ground sage, garlic powder and ground thyme.
Pour the cream mixture over the potatoes and toss gently. You want every potato to be coated! Line half of the potatoes in the baking dish – you can see I do white potatoes on one side and sweet on the other side. Sprinkle half of the breadcrumb/cheese mixture over top of the potatoes. Layer on the rest of the potatoes. Sprinkle the rest of the crumb/cheese mixture on top. I also pour any of remaining milk/cream over the potatoes, dispersing it evenly.
Bake the potatoes for 1 hour. Remove them and test to see if the potatoes are tender. If the crumbs on top get to brown, you can cover the pan with foil. Let them sit for a few minutes to settle before serving.
You can make these a few hours ahead of time and leave them covered at room temperature! Reheat right before serving.