Preheat your oven to 350°F (175°C).

In a large mixing bowl, combine ricotta cheese, finely chopped onion, a beaten egg, fresh parsley, and a sprinkle of salt and pepper. Stir in ½ cup each of mozzarella and Parmesan cheese until the mixture is creamy.

In another bowl, pour in your marinara sauce and add a splash of water, stirring to combine. Spread ¾ cup of this sauce evenly across the bottom of a 13x9-inch baking dish that you've lightly sprayed with cooking spray.

Carefully stuff each uncooked manicotti shell with the ricotta mixture. Place the filled shells in a single layer over the sauce in your baking dish.

Once all the manicotti shells are in place, pour the remaining marinara sauce generously over them.

Cover the baking dish tightly with aluminum foil and bake for 50 minutes.

After 50 minutes, carefully remove the foil and sprinkle the remaining mozzarella and Parmesan cheese over the top. Return the dish to the oven and bake uncovered for an additional 10-15 minutes.

Once your manicotti is beautifully golden and bubbling, take it out of the oven and let it rest for a few minutes before serving.