Preheat your oven to 375F and move the rack to the top third of the oven.
Boil a salted pot of water and cook the manicotti 2 minutes less than package directions indicate (they will finish cooking in the oven). Once the pasta is done, drain it and keep in your colander/strainer until needed.
Pour the jar of marinara sauce in a 9x13 casserole dish and spread it out evenly.
While the manicotti cooks, thaw the spinach and squeeze the water out thoroughly. Add it to a prep bowl along with the ricotta, parmesan, egg, salt & pepper, garlic powder, and Italian seasoning. Stir together until well combined.
Spoon or pipe the mixture into each manicotti noodle and add them to the casserole dish as you fill them. I find it fastest to use a small spoon (like a teaspoon), and I fill each one halfway then turn it around and finish filling it from the other opening. You can pack quite a lot of filling into each manicotti shell so it all gets used up (let some filling stick out a bit as shown in the step-by-step photos in the blog post).
Top with the mozzarella and cover the casserole dish tightly with foil.
Bake for 30 minutes, then remove the foil and broil for a few minutes to brown the cheese (watch it carefully so it doesn't burn).
Serve immediately with extra salt & pepper as needed and fresh basil and/or parsley if using.