Cook lasagna noodles according to package directions until al dente. Drain and set aside.
In a food processor or high-speed blender, combine soaked cashews, tofu, plant milk, nutritional yeast, lemon juice, garlic powder, oregano, salt, and pepper. Blend until smooth and creamy.
If using onion and garlic, sauté in a large pan until fragrant, about 3-4 minutes. Add marinara sauce and simmer for 5 minutes.
Spread a thin layer of sauce on the bottom of a 9x13 inch baking dish.
Layer with lasagna noodles, half the cashew ricotta, half the spinach, and one-third of remaining sauce.
Repeat with another layer of noodles, remaining ricotta, remaining spinach, and half of remaining sauce.
Top with final layer of noodles, remaining sauce, and all the vegan mozzarella cheese.
Cover tightly with foil and bake for 35 minutes.
Remove foil and bake an additional 10-15 minutes until cheese is melted and lightly golden.
Let rest for 10-15 minutes before slicing and serving.