Preheat oven to 375°F.
Cook ziti in salted water until al dente, then drain and set aside.
In a large pan, cook the sausage and onion until browned.
Stir in Italian seasoning, diced tomatoes, pasta water, marinara sauce, and balsamic vinegar. Let simmer for about 10 minutes.
In a bowl, mix together ricotta, eggs, parsley, 1 cup mozzarella, and parmesan.
Spread a little sauce on the bottom of a 9x13 pan. Add half of the pasta, all of the cheese mixture, half of the sauce, the rest of the pasta, the remaining sauce, and finish with the rest of the mozzarella on top.
Bake uncovered for 25–30 minutes or until hot, bubbly, and golden.