Preheat the oven to 375°F. Grease a 9x13inch baking dish.
In a large skillet, over medium-high heat, brown the sausage and onion, breaking it up with a spoon until the sausage is fairly fine. Drain any fat.
Add the pasta sauce, tomatoes, water, and Italian seasoning. Simmer uncovered for 12 minutes or until thickened, stirring occasionally. Season with salt and pepper to taste,
Meanwhile, boil ziti in a large pot of salted water until al dente. Drain well.
In a small bowl, mix ricotta, egg, parsley, 1 cup mozzarella cheese, and parmesan cheese.
Add 1 cup of sauce to the bottom of the prepared baking dish. Layer half of the ziti, top with all of the ricotta mixture, and half of the sauce.
Add remaining ziti, remaining sauce, and top with remaining mozzarella cheese.
Bake uncovered for 25-30 minutes or until golden and bubbly.