Preheat an oven, then grease a 9×13 baking dish with nonstick spray or butter. Fill a large bowl with cool water. Peel the potatoes, then place in the water to prevent oxidation. Work in batches as needed, then return the sliced potatoes to the water.
To the prepared baking dish, add a thin layer of potatoes. It’s fine if some overlap, just not too many. Season lightly with a generous pinch of kosher salt and freshly ground black pepper. Top with another layer or two of potatoes. Repeat until you’ve used them all.
Stop when the cream reaches the top of the dish. Whether you use it all will depend on how many potatoes are in the dish.
Carefully transfer to the oven then cook. The top of the gratin will turn deep golden brown as it cooks. Cool slightly (or you’ll burn your mouth!), then serve with a sprinkle of flaky sea salt and fresh thyme leaves.