Preheat the oven to 350F. Lightly grease an 8" x 11" baking dish with olive oil.
Bring a medium pot of water to a boil. Add the cauliflower florets to the pot and boil for 7 minutes, or until very fork-tender. Strain off the water.
Add the boiled cauliflower florets, filtered water, cashews, nutritional yeast, vinegar, sea salt, and black pepper to a blender (do not use a food processor).
Blend on high for 2 minutes, or until completely smooth and creamy. Set within reach of the stove.
Heat the olive oil and garlic in a large sauté pan over low heat. Cook for 1 minute, or until the garlic becomes soft and fragrant. Add the spinach and cook for 3 minutes, or until it begins to wilt.
Stir in the cauliflower sauce. Continue to cook for another minute or two, or until the sauce thickens ever so slightly. Turn off the heat.
Spread about one-quarter of the marinara into the bottom of the prepared baking dish. Add a layer of lasagna noodles. Cover the noodles with about half of the spinach alfredo sauce. Top with another one-quarter of the marinara sauce. Add another layer of lasagna noodles and cover them with the remaining spinach alfredo sauce. Top with another one-quarter of the marinara sauce. Finally, top with the last layer of lasagna noodles and cover with the remaining marinara sauce.
Cover the lasagna with foil and bake for 30 minutes. Then, remove the foil and sprinkle with as much of the vegan parmesan (if using) as desired and continue to bake for another 20 minutes.
Remove the lasagna from the oven and let it rest for at least 5 minutes before serving.
Leftovers can be refrigerated for up to 5 days or frozen for up to 1 month.