Peel, wash, and cut the potatoes into very thin slices using mandoline slicer or a knife.
Heat the heavy cream, milk, and butter over medium-high heat until the butter is melted. Season the cream with salt.
Grease a baking dish with butter. Layer the potato slices into the baking dish. Pout the cream sauce evenly over the potatoes. Add fresh thyme leaves or sprigs if desired. Bake, covered for 1 hour, then uncover and bake 30 minutes longer or until tender. Sprinkle with freshly ground black pepper before serving.
~ Free Tip ~Note: just in case, put your dish on a baking pan or a jelly roll pan. First time I made this recipe I didn't need it, but the second time the cream leaked over the rim. I think I sliced an extra potato and my dish was too crowded.