1. Cook pasta in a large pot of salted water, according to package directions, just until al dente. Drain, but do not rinse.
2. In a large bowl, mash the tofu, nutritional yeast, salt, onion powder, and oregano until evenly mixed.
3. Thaw froze spinach (if using), and blot off excess water. Roughly chop. Stir optional oil and spinach into the tofu ricotta.
4. Preheat your oven to 350° Fahrenheit or 176° Celsius.
5. Spread about a third of the tomato sauce into the bottom of a 9x13 inch pan.
6. Stuff the cooked pasta shells with tofu filling, then arrange on top of the sauce in the pan.
7. Evenly spread all remaining sauce over the shells. Sprinkle on a layer of vegan cheese if desired, and place the pan on the center rack of the oven.
8. Cover the top of the casserole dish with a thin baking sheet or a layer of tin foil. Bake 20 minutes.
9. Remove the cover and continue cooking another 5 minutes or until sauce is bubbly and shells are lightly golden.
10. While the vegan stuffed shells cook, I like to heat up additional sauce for serving.
11. Serve hot, or allow to cool fully before covering and refrigerating or transferring the shells to meal prep containers and freezing leftovers for later.