Preheat the oven to 350F and lightly oil a large, rectangular (9" x 13" x 2.5") baker.
Bring a large pot of salted water to boil. Add the lasagna sheets. Cook according to package instructions (probably 10-12 minutes), or until tender.
While the water heats and the lasagna cooks, place the cashews into a food processor fitted with the S blade. Add two tablespoons water, lemon juice, nutritional yeast, salt, garlic powder, and black pepper. Pulse a few times, then process for about a minute, to break the cashews down. Crumble the tofu into the processor. Continue processing for another 2 minutes, stopping a few times to scrape the sides of the processor down. When the ricotta is completely creamy and smooth, stop the processor. Taste and adjust the salt/pepper/lemon to your liking.
Transfer the ricotta to a mixing bowl and add the chopped spinach. Mix well.
Place about 3/4 cup of the marinara sauce into the prepared baking dish, then spread it evenly around the bottom of the dish.
When the pasta is ready, line a work surface with parchment, foil, or a baking sheet. Spread a cooked lasagna noodle over the surface. Top with 1/4 cup of the ricotta mixture and spread the mixture evenly over the noodle. Add a tablespoon of the marinara sauce and cover the top of the ricotta lightly. Starting at the short, bottom end of the noodle, roll the lasagna noodle up. Place the lasagna roll into the baker. Repeat with your remaining noodles, arranging the rolls in your baking dish in rows. When the baking dish is full, top the rolls with remaining marinara sauce, enough to cover them generously.
Cover the baking dish with foil and transfer it to the oven. Bake for 15 minutes, covered. Remove the foil and bake for another 15-20 minutes, or until the sauce is darkened, concentrated, and bubbling at the edges. Serve!