Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish and set a baking sheet aside to hold the cooked shells.
Bring a large pot of salted water to a boil and par-cook the jumbo shells according to package instructions for stuffed shells; if no guidance is given, boil for half the package time plus two minutes.
Drain the shells and rinse with cold water to stop cooking, then spread them on the baking sheet so they don’t stick while you prepare the filling.
Make the vegan ricotta: in a food processor combine the drained tofu, soaked cashews, nutritional yeast, garlic, lemon juice, lemon zest, Italian seasoning, kosher salt, and a few grinds of black pepper; process until smooth and creamy, about 45–60 seconds.
Transfer the ricotta to a bowl and fold in the thoroughly drained spinach and the chopped fresh basil (about 1/2 cup loosely packed chopped basil) if using.
Spread 1 cup of marinara sauce 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 1 1/2 tablespoons per shell), place each filled shell cut side up in the prepared dish, and repeat until all shells are filled.
Pour the remaining marinara over the filled shells and sprinkle the dairy-free Parmesan on top if using.
Bake on the middle rack for 30–35 minutes, until bubbly and lightly browned on top. Let rest a few minutes before serving.
