Preheat the oven to 350°F.
Cook the ziti pasta according to package directions until al dente, then drain and let cool slightly.
While the pasta cooks, brown the Italian sausage in a skillet, breaking it up as it cooks. Chop into bite-sized pieces and set aside.
In a large bowl, combine the ricotta cheese, egg, parsley, 1 1/2 cups of shredded mozzarella cheese, and parmesan cheese. Mix until well blended.
Add the cooked sausage and pasta to the cheese mixture, stirring to combine evenly.
Pour in about 3/4 of the pasta sauce and mix until everything is coated.
Transfer the mixture into a 2-quart casserole dish, spreading evenly.
Pour the remaining pasta sauce over the top and sprinkle with the reserved 1/2 cup mozzarella cheese.
Bake in the preheated oven for 30 to 35 minutes until the cheese is melted, bubbly, and golden around the edges.
For freezing: cover with foil before baking. To serve, thaw completely, bake covered at 350°F for 20 minutes, then uncover and bake an additional 10-15 minutes until heated through.