Preheat your oven to 350 degrees F.
Thoroughly wash the russet potatoes to remove all surface dirt. Scrub with a vegetable brush if necessary. For uniform baking it is important for each potato to be approximately the same thickness, but in no case should they be less than 1-1/2 inches thick.
Hold a potato on its side and carefully slice the potato lengthwise into three equal width slices.
Place each slice (12 total for the 4 potatoes used) on a large non-stick baking sheet and bake in the oven for about 40 minutes until the inner portions of the potatoes are soft and the cut surfaces are beginning to brown. While the potato slices are baking, wash and peel the onions and garlic. Dice the onions, crush the garlic, and put into a covered baking dish. Mix in the pepper and cook on "high" in your microwave oven for about 3 to 4 minutes until the onions start to become translucent.
When the onions and garlic are cooked, add the crushed tomatoes and oregano, mix, cover, and cook for about another 30 minutes (mixing every 10 minutes) until the sauce thickens. A little trick to aid during the thickening process which allows evaporation without splashing: place the end of a wooden toothpick between the lid and the baking dish, with one end sticking out so you can hold it while removing the lid. When the sauce has thickened to the desired consistency, remove from the microwave oven.
While the sauce is cooking, open the can of beans and pour off the liquid. Place the beans in the container of your blender, add the lemon juice, nutritional yeast, and corn starch, and run the blender at high speed until the contents are creamy smooth.
Wash, peel, and slice the onion for the topping.
When the potatoes are baked, remove from the oven and set on a wire rack. The sauce and the bean cream should also be ready. As quickly as possible, spread a layer of the bean cream on each potato slice. Carefully spoon on the tomato sauce and top with the onion rings (each step is shown in the photo to the left). Place back in the oven and turn up the heat to 400 degrees F. for approximately another 20 minutes.
When the pizza potatoes are baked, remove from the oven, and set on a wire rack. When the pizza potatoes have partially cooled, they can be picked up and eaten as "finger food.".