Preheat the oven to 400F. Lightly grease a 9 x 13 baking dish.
Thinly slice the potatoes into ⅛-inch-thick rounds using a knife or mandolin (be very careful if using a mandolin).
To a high-speed blender, add the cauliflower, water, cashews, nutritional yeast, apple cider vinegar, garlic, and sea salt, and blend until smooth. Set aside.
Arrange about two-thirds of the potatoes in the pan, creating two shingle-like layers. Pour about two-thirds of the cauliflower cream sauce over top and use a spatula to gently spread it over the potatoes. Top with the remaining potatoes (about one layer) and pour the remaining cream sauce over top and use a spatula to gently spread it over the potatoes. Season the top with black pepper and sprinkle with the rosemary.
Bake for 45 to 55 minutes, or until golden brown.
Let cool slightly, and then slice and serve.