Butter a shallow 2 1⁄2-quart baking dish (no more than 2 inches deep). Peel potatoes and slice them using a food processor fitted with the thinnest slicing blade. Thinly slice apples into rounds, discarding any seeds. Bring cream and milk to a boil in the skillet. Add 3⁄4 teaspoon salt, potatoes, and apples and simmer, partially covered, gently turning occasionally (making sure potatoes aren’t sticking together) until nearly tender but not breaking apart, 12 to 15 minutes. Spoon a third of the potatoes and apples into the prepared baking dish and top with half of onions. Repeat, then top with a layer of remaining potatoes and onions, pouring any remaining cream on top. Bake on a rimmed baking sheet until bubbling and golden brown, 30 to 35 minutes.