Cook pasta: Bring a large pot of salted water to a boil. Cook manicotti shells until just al dente (slightly firm, since they’ll bake further). Drain and drizzle with olive oil to prevent sticking.
Prepare filling: In a large bowl, combine ricotta, ½ cup mozzarella, Parmesan, egg, spinach, garlic, Italian seasoning, salt, and pepper. Mix until creamy.
Fill shells: Spoon mixture into a piping bag or zip-top bag with the corner cut off. Carefully fill each manicotti shell.
Assemble: Spread 1 cup marinara sauce over the bottom of a 9×13-inch baking dish. Arrange filled manicotti shells in a single layer. Cover with remaining sauce and sprinkle with reserved mozzarella cheese.
Bake: Cover with foil and bake at 375°F (190°C) for 25 minutes. Remove foil and bake an additional 10 minutes, until cheese is golden and bubbly.
Serve: Garnish with fresh basil or parsley. Serve hot with garlic bread or a crisp salad.
