In a small skillet, saute onion in oil for 3 minutes. Add garlic; cook 1 minute longer. Set aside.
In a large bowl, combine the ricotta, 1/2 cup mozzarella cheese, cream cheese, 4 tablespoons Parmesan cheese, Italian seasoning and pepper; beat until smooth. Stir in onion mixture and spinach. Stuff into the manicotti shells.
Pour half of the spaghetti sauce into a greased 13x9-in. baking dish. Arrange shells over sauce; top with remaining sauce.
Cover and bake at 350° for 25 minutes. Uncover; sprinkle with remaining mozzarella and Parmesan cheeses. Bake 5-10 minutes longer or until cheese is melted.