Bring a large pot of salted water to a boil and cook the jumbo shells according to package directions until al dente; drain and rinse with cool water to stop cooking, then set aside to cool.
Preheat the oven to 350°F (175°C) and lightly grease a 9 x 13-inch baking dish.
In a large bowl, combine the ricotta, shredded mozzarella, 3/4 cup grated Parmesan, eggs, dried oregano, salt, black pepper, and the well-drained spinach; mix until evenly combined.
Spread about 1 cup of the marinara sauce in the bottom of the prepared baking dish to coat the surface.
Spoon about 2 tablespoons of the cheese-and-spinach filling into each cooled shell, then place the filled shells in the baking dish in a single layer.
Pour the remaining marinara sauce evenly over the filled shells and sprinkle additional grated Parmesan over the top if desired.
Cover the dish with aluminum foil and bake for about 35 minutes, until heated through and bubbly; remove foil for the last 5 minutes if you want a slightly browned top.
Let the casserole rest for 5 minutes before serving.
