Make the whipped basil tofu ricotta: Add the tofu, ½ cup of fresh basil leaves, lemon juice, miso paste, nutritional yeast, soy milk, Italian seasoning, salt (if using), and black pepper to a large food processor. Pulse until the ingredients are well-combined and the mixture has a soft, ricotta-like texture. Stop before it gets too smooth and liquidy—it should be easily spreadable with some visible lumps and bumps.
Prepare the lasagna: Preheat the oven to 400°F. Spread 1 cup of marinara sauce on the bottom of a 9x13-inch casserole dish. I like to use a ladle to spoon the sauce. You can then use the bottom of the ladle to smooth it out, distributing it evenly.
Place 3 lasagna noodles side-by-side on top of the sauce. Top them with an even layer of the sliced zucchini.
Cover the zucchini layer with 1 cup of marinara sauce, smoothing it out evenly with the back of a ladle. Next, gently rest 3 lasagna noodles in the sauce, then cover them with the whipped basil tofu ricotta in an even layer.
Place 3 more lasagna noodles on top of the ricotta. Evenly distribute another layer of marinara sauce over the noodles. Spread the yellow squash slices (or additional zucchini slices) over the top.
Cover the yellow squash slices with the last 3 lasagna noodles. Then, smooth the rest of the marinara sauce over the top.
Pour ½ cup of water around the edges of the lasagna. This will prevent the sides of the noodles from getting hard and crunchy in the oven.
Cover the casserole dish with aluminum foil and bake for 40 to 45 minutes, until noodles are soft and the squash is fully cooked. You may need 45 minutes or longer if your squash slices are on the thicker side.
Remove the lasagna from the oven and let it cool, uncovered, on a potholder or thick towel for about 10 to 15 minutes, until it reaches an appropriate temperature for serving. Cut it into 9 large servings or 12 smaller servings. Serve immediately, garnished with fresh basil leaves, lemon zest, and/or vegan Parmesan, if desired.