1. In a skillet, cook sausage, onion and garlic until pork is no longer pink and onion is tender; drain.
2. Add marinara sauce and oregano; simmer for 5 minutes.
3. In a bowl, combine ricotta cheese, egg, ¼ cup Parmesan and spinach.
4. In a greased 13-in. x 9-in. x 2-in. baking dish, spread 1 cup meat sauce.
5. Arrange 3 noodles over sauce.
6. Spread one-fourth of the ricotta cheese mixture over the noodles, top with 1 cup of meat sauce.
7. Sprinkle with ½ cup mozzarella cheese. Repeat process 3 times.
8. Top with remaining Parmesan cheese.
9. Bake, uncovered, at 350°F for 40-45 minutes.
10. Let stand for 10 minutes before cutting.