Preheat oven to 350 degrees
Bring a large pot of water to a boil and add the manicotti noodles. Cook for 9 minutes or per instructions on box, until just done. Drain and rinse with cold water so they are only slightly warm to the touch. Set aside.
Heat a large skillet over medium heat. Add olive oil and when hot sauté onion until translucent, approximately 4-5 minutes. Add the spinach and cook until completely wilted and the water has been cooked away. Set aside to cool.
Add the cashews to a food processor and on low setting process until fine and crumbly. They should start to stick together slightly on the sides.
Add remaining ingredients to the food processor, except the onions and spinach. Process until smooth, occasionally scraping the sides as needed. Now add the onion and spinach mixture and process until the mixture is nicely blended.
In a large glass baking dish, spread 1 cup of the sauce on the bottom to create a thin layer.
Stuff each individual manicotti shell with the veggie ricotta mixture. Either use a spoon or my preferred method is to place the mixture in a strong, sealable quart size bag (e.g. Ziploc), clip the corner, and use the bag to pipette the mixture into the noodles. Fill noodle halfway and then repeat on the other side.
Place each stuffed noodle in the baking dish. You may have a couple noodles that break which is ok, there is usually a couple extra. When complete spread remaining sauce evenly on the top of the noodles.
Bake, uncovered, for 25-30 minutes until the sauce is bubbly and has darkened in color slightly. Garnish with chopped basil if desired. Use a spatula to serve and enjoy.