Preheat oven to 350°F and grease a 2-quart casserole dish.
Cook ziti pasta according to package directions until al dente. Drain and set aside.
Cook Italian sausage in a large skillet, breaking it up as it cooks. Drain excess fat and chop into bite-sized pieces.
In a large bowl, mix together ricotta cheese, egg, parsley, 1½ cups mozzarella cheese, and parmesan cheese.
Add the cooked pasta and sausage to the ricotta mixture, stirring to combine.
Mix in ¾ of the pasta sauce, reserving the remainder for topping.
Transfer the mixture to the prepared casserole dish.
Top with remaining pasta sauce and the remaining ½ cup mozzarella cheese.
Bake for 30-35 minutes, or until hot and bubbly throughout.
Let rest for 5 minutes before serving.