Preheat the oven to 350°F.

In a skillet, cook sausage, onion, and garlic until pork is no longer pink and onions are tender; drain. Add sauce; simmer for five minutes.

In a bowl, combine ricotta, egg, ¼ cup of grated parmesan, oregano, and spinach.

In a greased 13x9 baking dish, arrange ingredients in layers as follows: Half of sauce mixture, ziti noodles, all of ricotta mixture, remaining sauce, mozzarella, and remaining parmesan. Bake uncovered for 40-45 minutes.

Serve and enjoy!