Preheat the oven to 350 degrees Fahrenheit.
Make the filling by placing the cashews in a blender or food processor with ½ cup water and process to a very smooth paste. Add the beans to the blender along with the nutritional yeast, onion powder, garlic powder, oregano, nutmeg and salt and ground black pepper to taste. Process until you have a smooth filling that should be stiff, not runny, so you can easily stuff the manicotti. Try not to use any more water while processing, but if your blades won't move, add a tablespoon or two.
Heat oil in a saucepan. Add the onions and garlic and saute along with some salt and ground black pepper and saute until the onions turn soft but not brown. Add the spinach and saute until the spinach is soft and tender. Turn off the heat.
Let the spinach mixture cool slightly, then stir in the cashew filling, half the mozzarella shreds and parsley. Mix well.
Coat the bottom of a 13 by 9 inch baking dish with some cooking spray and ladle half the marinara (about 1 ½ cups) into the dish, spreading it evenly.
Fill each manicotti cylinder with the stuffing and then place in the baking pan in a single layer on top of the marinara. My baking dish is a bit narrow, so I arranged them diagonally, which also looked really pretty.
Spoon over the remaining marinara to cover the manicotti, and sprinkle on the remaining mozzarella shreds. You can drizzle on some olive oil at this stage to help add some moisture and richness to your dish, but it's optional.
Bake for 35 minutes. Let stand 10 minutes before serving.