While the pasta is boiling, add the tofu, lemon, nutritional yeast, onion powder, garlic powder, and salt to a food processor. Pulse until everything is combined and the texture resembles ricotta cheese. Fold in the frozen chopped spinach.
Once the pasta is done boiling, drain and rinse under cool water. Normally you wouldn't want to rinse pasta as the starch from the pasta water helps it to hold on to the sauce. However, since we will need to handle the shells, we do not want to burn our fingers.
After rinsing the pasta, using a spoon, spoon the ricotta filling into the shells.Grease a 9x11 baking dish and pour half of the jar of marinara sauce on the bottom of the dish to coat the dish.Layer the filled pasta shells on top and then add the rest of the marinara sauce on top.