Preheat your oven to 350°F. Lightly coat a 13x9-inch baking dish with cooking spray.
In a bowl, combine ricotta, onion, egg, parsley, salt, and pepper. Stir in 1/2 cup each of mozzarella and Parmesan.
In another bowl, mix marinara sauce with water. Spread 3/4 cup of this sauce onto the bottom of your prepared dish.
Fill uncooked manicotti shells with the prepared cheese mixture. Arrange these over the marinara sauce in the dish.
Pour the remaining marinara sauce over the shells, ensuring even coverage.
Cover with foil and bake for 50 minutes, or until pasta is tender. Remove foil and sprinkle the remaining mozzarella and Parmesan on top.
Bake uncovered for another 10-15 minutes, until the cheese is melted and bubbly.
Garnish with parsley if desired, and serve hot.