Preheat the oven to 375°F and grease a 9x13-inch baking dish; set a baking sheet aside to place shells on after cooking.
Bring a large pot of salted water to a boil and par-cook the jumbo shells following package "stuffed shell" instructions; if no guidance, cook for half the recommended time plus 2 minutes.
Drain the shells carefully, rinse with cold water to stop cooking, and spread them on the baking sheet to prevent sticking.
Make the vegan ricotta: combine the tofu, soaked cashews, nutritional yeast, garlic, lemon juice, lemon zest, Italian seasoning, kosher salt, and black pepper in a food processor and process until smooth and creamy, about 45–60 seconds.
Transfer the blended ricotta to a bowl and fold in the drained spinach and chopped fresh basil (about 1/2 cup loosely packed basil from the ingredients).
Spread 1 cup of marinara sauce evenly across the bottom of the prepared baking dish.
Fill each par-cooked shell with the ricotta mixture using a spoon, cookie scoop, or piping bag (about 1½ tablespoons per shell depending on shell size), then place each shell cut-side up in the baking dish.
Pour the remaining marinara over the filled shells and sprinkle dairy-free parmesan on top if using.
Bake on the middle rack for 30–35 minutes, until sauce is bubbly and the tops are lightly browned.
Remove from oven, top with additional fresh basil if desired, and serve warm.