Soak the cashews in boiling water for 1 hour, strain. Alternatively, soak the cashews in room-temperature water overnight. Cover and store in the fridge, strain.
Make the cashew cheese sauce: In a blender add the soaked cashews, almond milk, garlic, sea salt, nutritional yeast, Dijon mustard, and apple cider vinegar. Blend until smooth and creamy, about 1 minute. (If mixture is too thick, add splashes more almond milk. It should be a smooth and pourable consistency).
Preheat oven to 350F/180C. Lightly paint a 10-inch cast iron skillet with avocado oil and sprinkle with salt and dried rosemary. Arrange the potatoes side-by-side to stand up vertically on a slight angle; working your way around the edge of the pan and into the centre. Wedge thin slices of onion between the potatoes.
Pour the cashew cheese sauce over the potatoes ensuring it falls between the nook and crannies. Pour just enough cashew cheese sauce to cover all the potatoes, but still ensuring about the tops are still exposed.
Cover the pan with tin foil, or an oven safe lid and bake until the potatoes are fork tender, 60-75 minutes. Remove the foil and turn oven to broil, cook for another 3 to 5 minutes to lightly brown the top.