Make 1 pie crust according to pie crust recipe instructions.
Preheat oven to 375ºF
Heat a large saute pan over medium heat with olive oil. Dice your onion and add to heated pan. Saute until they are just beginning to brown slightly.
Peel your red potatoes and cut them into fours. Pulse in your food processor until very finely chopped similar to hash browns. Add potatoes to the pan with onions and saute stirring occasionally for about 10 mins or until they are tender.
Set potatoes and onions aside in a bowl to cool. Chop potatoes and mushrooms.
Optional: Saute mushrooms in a pan until slightly brown and fragrant. This will give them a much richer meatier flavor.
Mix your potatoes and onions with your vegetables and spread evenly into your pie crust.
In a high powered blender, blend your water, soaked cashews, garbanzo bean flour, nutritional yeast, salt, pepper, and other optional seasonings if you choose. Blend until smooth and liquidy. Pour evenly into your pie pan topping off your other ingredients.
Bake in the center of the oven for 30 - 45 minutes or until a knife inserted in the center comes out dry. Let cool for at least 20 minutes before slicing. If you want clean cuts, refrigerate before slicing then reheat individual slices when you want to eat them.