Preheat oven to 400 degrees F.
Add the tomato sauce, 1/2 teaspoon garlic powder, dried minced onion, Italian seasoning, basil, oregano, salt and pepper to a medium saucepan. Bring to a simmer then add in the Parmesan cheese. Reduce heat to low and continue to cook.
Bring a large pot of water to a boil. Add the jumbo pasta shells and cook 2 minutes less than the package directions call for. Drain the pasta and let it cool.
In a medium bowl add the ricotta cheese, mozzarella cheese, spinach and 1/4 teaspoon garlic powder. Mix together.
Add 1 cup of marinara sauce to an 8x8 baking dish.
Fill each of the shells with the spinach mixture and add the shells to the baking dish.
Pour the remaining marinara sauce over the shells. Cover with aluminum foil and bake for 25 minutes.