Preheat oven to 350°F (175°C).
Place raw cashews in a heat-safe bowl. Pour 2 cups of boiling water over them. Let soak 10–15 minutes, then drain.
Cook jumbo shells according to package directions, but remove them 1 minute early so they are very al dente. They will finish cooking in the oven. Set aside on a plate to cool.
Add drained cashews to a high-speed blender with tofu, lemon juice, nutritional yeast, salt, pepper, and oat milk. Blend on HIGH for 2–3 minutes, scraping down the sides as needed, until smooth and creamy. Taste and adjust seasoning. Add more oat milk if the mixture is too thick.
Pour the ricotta mixture into a medium bowl and stir in the squeezed, thawed spinach until fully combined.
Spread half of the marinara sauce evenly over the bottom of a 10x7 casserole dish (or similar size).
Spoon the ricotta-spinach filling generously into each shell and nestle them into the marinara sauce, open side up. Spoon the remaining marinara over the top.
Top with dairy-free shredded cheese if using.
Cover the dish tightly with aluminum foil and bake for 30 minutes, or until sauce is bubbling.
If you added cheese, remove foil for the last 5–10 minutes to allow it to melt.
Serve warm, topped with fresh basil if desired.