Preheat oven to 375°F, lightly grease a 13”x9” pan with cooking spray. Set aside.
In a skillet over medium heat add the sausage and diced onion. Cook, breaking up the sausage as you go. Once sausage is fully cooked. Drain fat, if any.
Add the marinara, diced tomatoes, Italian seasoning, and water to the skillet. Bring to a simmer and simmer for 10 minutes.
Meanwhile, cook the ziti pasta to 1 minute under al dente. Drain the pasta and set aside.
In a medium sized bowl combine the ricotta, parsley, egg, and 1 cup of mozzarella and 1/2 cup parmesan. Stir to combine.
Add 1 cup of the sauce to the bottom of the baking dish. Add half of the ziti pasta to the dish. Top with all of the ricotta, doing your best to spread it in an even layer. Top with half of the remaining sauce.
Add the rest of the ziti, the remainder of the sauce, and then top with 2 cups of shredded mozzarella.
Bake for 20 minutes or until warm and bubbly.