Cook 24 jumbo shells in boiling water according to package directions until al dente. Drain and rinse immediately when done cooking. Set aside.
Preheat oven to 350F.
Place half the tofu, nuts, nutritional yeast flakes, lemon juice, and water in the food processor or blender. Process until smooth and creamy.
Crumble the other half of the tofu in a medium-sized mixing bowl. Mix in blended ricotta from food processor. Stir in basil, garlic powder, and fresh or frozen spinach.
Pour 1/2 to 2/3 of a jar of marinara sauce into a 9x13 casserole dish.
Carefully fill each shell with 2-3 tablespoons of ricotta filling. This ricotta recipe makes enough to fill each shell moderately, but not overflowing. Arrange shells in casserole dish with 4 shells across the short direction and six shells down the long direction.
When finished filling shells, top with remaining marinara. Add additional marinara if desired.
Top with vegan shredded mozzarella (link to recipe in blog post) if desired.
Bake for 30 minutes.