Peel the potatoes and slice them into 1/8-inch rounds. Set aside in a bowl of cold water if not using immediately.
Place the cashews, vegetable broth, almond milk, nutritional yeast, and salt in a high-speed blender. Blend until very smooth to create a creamy sauce.
Spread half of the sliced potatoes evenly in the prepared baking dish. Pour half of the blended sauce over the potatoes, then distribute the sautéed onions and garlic evenly on top.