Bring a large pot of salted water to a boil and cook the 8 ounce manicotti shells according to the package instructions until they are al dente. Drain the pasta and set it aside to cool slightly.
In a mixing bowl, combine the 15 ounce ricotta cheese, 10 ounce frozen spinach chopped, 1 medium onion , minced 2 cloves garlic (or garlic powder), 1 teaspoon Italian seasoning, ⅛ teaspoon nutmeg , 1 large egg, ½ teaspoon Kosher Salt and ½ teaspoon 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.
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 your air fryer temperature to 320 degrees F for 3-5 minutes.
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 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.
While the manicotti are air frying, warm additional marinara sauce in a separate saucepan on the stove or the microwave.
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 the air-fried manicotti with additional grated Parmesan cheese and chopped fresh parsley before serving.