Begin by boiling a large pot of salted water. Once boiling, carefully add the manicotti shells and cook them for about 7-8 minutes, or until they are al dente. Drain the shells and let them cool slightly.
In a large mixing bowl, combine the shredded chicken, chopped spinach, ricotta cheese, shredded mozzarella, grated Parmesan, and the egg. Season with garlic powder, onion powder, salt, and pepper. Mix until well combined.
Preheat your oven to 375°F (190°C).
Using a spoon or piping bag, carefully stuff each manicotti shell with the filling.
Grease your baking dish with olive oil. Spread 1 cup of marinara sauce on the bottom of the dish. Arrange the stuffed manicotti in a single layer and pour the remaining marinara sauce over the top. Sprinkle any extra mozzarella and Parmesan cheese over it.
Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and lightly browned.
Let the manicotti cool for a few minutes before serving. Garnish with fresh basil or parsley if desired.