Preheat oven to 375°F. Grease a 9x9" inch pan or medium-sized casserole dish.
Using a sharp knife or mandolin slicer, slice potatoes to 1/8" thickness. You may choose to hold potatoes in water while prepping to keep them from turning brown.
Puree all remaining ingredients into a smooth sauce. Taste and adjust spices to preference.
Spread onions along the bottom of your baking dish. Arrange potatoes in dish. (I like to stand the potatoes up, working from the edges to the center, but you can also just lay them on top of each other.) Pour "cheese" sauce over potatoes.
Cover dish with foil or a lid and bake for 45-60 minutes in preheated oven until potatoes are tender. Remove foil during last 10 minutes of baking to allow the edges to brown slightly (you can also sprinkle on some of your favorite vegan cheese shreds at this time). Remove from oven and allow to cool 5 minutes before serving.