Cook the Manicotti Pasta: Bring a large pot of salted water to a boil and cook the manicotti pasta tubes according to the package instructions until they are al dente. Drain the pasta and set it aside to cool slightly.
Prepare the Filling: In a mixing bowl, combine the ricotta cheese, spinach, diced onions, minced garlic (or garlic powder), Italian seasoning, ground nutmeg, egg, Kosher salt, and black pepper. Slowly fold in 1 cup of the mozzarella cheese and 1/2 cup of the Parmesan cheese. Mix everything well until the filling is evenly combined.
Fill the Manicotti: Carefully stuff the cooked manicotti pasta tubes with the prepared filling mixture using a spoon or piping bag. Gently press the filling inside the tubes to ensure they are evenly filled.
Preheat the Air Fryer: Preheat your air fryer to 320 degrees F for 3-5 minutes.
Arrange the Stuffed Manicotti: Spread about 1 cup of the marinara sauce to the bottom of the casserole dish. The dish should be slightly smaller than your air fryer basket to circulate the hot air around the baking dish. Add the stuffed manicotti to the marinara sauce and the remaining sauce.
Air Fry the Manicotti: Air fry the stuffed manicotti at 320 degrees F for a cook time of 10-15 minutes or until they are heated through, and the filling is hot and melty. Cooking times may vary depending on your air fryer model, so keep an eye on them to avoid overcooking.
When the manicotti is almost cooked, top with the remaining mozzarella and parmesan cheese.
Warm the Marinara Sauce: While the manicotti are air frying, warm additional marinara sauce in a separate saucepan on the stove or the microwave.
Serve: Once the manicotti are done, remove them from the air fryer and serve them hot. Pour the warmed marinara sauce over the top or dip it on the side.
Garnish (Optional): Garnish the air-fried manicotti with additional grated Parmesan cheese and chopped fresh parsley before serving.