Prep: Pre-heat the oven to 375F. Grease a 9x13" baking dish and set aside. Set a baking sheet aside as well.
Par-cook the Pasta: Bring a large pot of salted water to a boil and par-cook the jumbo shells according to the stuffed shell cooking instructions on the package. (If your shells don’t have this, cut the cooking time of the pasta in half, then add two minutes). Once the noodles have finished cooking, carefully drain them and rinse with cold water, then transfer them to the baking sheet to prevent sticking.
Make the Ricotta: Add the tofu, cashews, nutritional yeast, garlic, lemon juice and zest, Italian seasoning, salt, and black pepper to a food processor with an S-blade. Process until smooth and creamy, 45 to 60 seconds. Transfer the ricotta to a bowl and fold in the spinach and freshly chopped basil.
Stuff the Shells: Spread 1 cup of marinara sauce across the base of the baking dish. Use a spoon, cookie dough scoop, or piping bag to fill each shell, gently close it, then place it cut side-up in the pan. The amount of ricotta needed will vary on the size of the shell; I was able to fit around 1 1/2 tablespoons of filling in each shell. Repeat with the remaining shells.
Bake: Cover the shells with the remaining marinara and cheese, if using. Bake in the middle rack of the oven for 30 to 35 minutes, until browned and bubbly. Top with additional fresh basil if desired, then serve warm. Store leftovers in the fridge for up to 5 days.