Shred the potatoes with a box grater or another shredding tool. Then place them in a large bowl and cover them with cold water. Let that soak for at least 20 minutes. Then drain the shredded potatoes and rinse well with cold water to remove excess starch. Then transfer to a clean and dry dish towel and pat them dry.
Preheat the oven to 425℉/218℃ and grab an oven-safe baking dish or casserole dish.
Shred the cauliflower and towel dry those too, to get out any excess moisture. Add them to a bowl with the dry potato shreds and the finely chopped onion and pepper.
Blend all the cheese sauce ingredients in a high-speed blender until smooth. Then pour the mixture in the bowl of shredded veggies and mix well.
Transfer the mixture to your oven-safe dish and bake for 30 minutes. Then broil the dish for 2-3 minutes (watching carefully) to brown the top. Remove from the oven and serve warm.
Notes
For the best texture, don't skip the extra step to soak the potatoes.
If you don't have a high-speed blender, soak your cashews in hot water for 15-30 minutes and then drain them, before making the vegan cheese sauce.
Do not overbake the casserole as this can result in an overly soft texture.
Watch CAREFULLY when broiling to prevent burning your dish.
You may use 3 lbs of potatoes and skip the cauliflower if preferred.
For nut-free use sunflower seeds or pepitas instead of cashews with similar results.
Although you MAY substitute the tapioca starch for corn starch or flour, I don't recommend it unless necessary. The tapioca starch is a unique ingredient that gives the vegan cheese sauce its stretchy texture.