Preheat your oven to 375°F (190°C).
In a large pot, bring salted water to a boil. Cook the large pasta shells according to the package instructions until al dente. Drain and set aside to cool.
In a mixing bowl, combine the ricotta cheese, chopped spinach, half of the mozzarella cheese, grated Parmesan cheese, egg, garlic powder, onion powder, salt, and pepper. Mix thoroughly until well combined.
Spread a thin layer of marinara sauce on the bottom of the baking dish to prevent sticking.
Stuff each pasta shell generously with the spinach and ricotta mixture and place them in the baking dish seam-side up.
Pour the remaining marinara sauce over the stuffed shells, covering them completely. Sprinkle the remaining mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
After 20 minutes, remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Once done, remove from the oven and let it sit for a few minutes before serving. Garnish with fresh basil if desired.