Heat oven or smoker to 400F degrees.
Slice each potato into 1/8-inch slices.
Stack the potatoes into 2-3-inch stacks, alternating potato colors.
Grease a round baking dish or cast iron pan.
Transfer the potato stacks to the pan, laying them on their sides and creating an outer and inner circle.
In a sauce pot, melt the butter over medium heat. Whisk in the flour. Cook for three minutes, whisking occasionally. Add the rosemary, salt, pepper and nutmeg.
Whisk in the cream and milk. Continue whisking, until it starts to thicken.
Remove the pan from the heat, and whisk in the cheese.
Pour the sauce over the potatoes. Cover with foil, and place on a sheet pan.
Bake in a 400F-degree oven or smoker for one hour. Remove the foil, and bake 20-30 more minutes, or until the cheese begins to brown and bubble.