Preheat the oven to 350°F (180°C or 160°C for a fan oven), and lightly grease a 9x13-inch (33x22 cm) baking dish.
Cook the pasta according to package instructions. Drain and set aside.
Meanwhile, in a large skillet or dutch oven, heat olive oil over medium high heat, add the italian sausage and onion, and cook breaking the sausage apart with a spatula until the sausage is browned (about 7-8 minutes).
Drain excess fat if needed.
Add garlic, and cook for 1 minute.
Add marinara sauce, diced tomatoes, water, italian seasoning, salt and pepper. Bring the mixture to a boil, reduce heat to a simmer and cook for 10 minutes stirring occasionally.
In a medium mixing bowl, combine ricotta cheese, egg, half of the mozzarella cheese, parmesan, parsley, salt, and pepper.
Add about 2 cups of meat sauce to the ziti, and stir together.
To the baking dish, add all of the ziti, spread the ricotta filling on top.
Top with the remaining meat sauce, and remaining mozzarella cheese.
Cover the casserole loosely with foil (make sure it’s not touching the cheese), and bake in the preheated oven for 20 minutes. Remove the foil, and bake for 10 more minutes. If the top is not slightly browned, place under the broiler for 2-5 minutes until it is golden.
Garnish the baked ziti with fresh chopped parsley before serving if desired.