Cook the Pasta: Bring a large pot of salted water to a boil. Cook the jumbo shells until just al dente (about 8 minutes). Drain, toss with olive oil, and set aside.
Make the Filling: In a mixing bowl, combine ricotta, mozzarella, Parmesan, spinach, egg, garlic, Italian seasoning, salt, and pepper. Stir until well blended and creamy.
Stuff the Shells: Using a spoon, carefully fill each pasta shell with 2–3 tablespoons of the cheese mixture.
Prepare the Baking Dish: Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish. Arrange stuffed shells in a single layer over the sauce.
Top & Bake: Pour remaining marinara sauce evenly over the shells, sprinkle with the extra mozzarella, and cover the dish with foil. Bake at 375°F (190°C) for 25 minutes, then uncover and bake an additional 10 minutes until cheese is melted and bubbly.
Rest & Serve: Allow the dish to rest for 10 minutes before serving. Garnish with fresh basil or parsley for extra flavor and presentation.
