Prepare the Pasta:

• Preheat the oven to 375°F (190°C).
• Cook the jumbo pasta shells in a large pot of salted boiling water until al dente, according to package instructions. Stir occasionally to prevent sticking.
• Drain the shells and rinse under cold water to stop the cooking process. Set aside.

Prepare the Filling:

• Heat the olive oil in a large skillet over medium heat.
• Add the fresh spinach and sauté until wilted, about 2-3 minutes.
• Remove from heat, let it cool slightly, then chop the spinach finely.
• In a large bowl, combine the chopped spinach, ricotta cheese, egg, 1/2 cup of mozzarella cheese, Parmesan cheese, minced garlic, dried oregano, salt, and black pepper. Mix until well combined.

Assemble the Dish:

• Spread 1 cup of marinara sauce evenly over the bottom of a 9×13-inch (23×33 cm) baking dish.
• Fill each cooked pasta shell with approximately 2 tablespoons of the ricotta-spinach mixture.
• Arrange the stuffed shells in the baking dish in a single layer.
• Pour the remaining 1 cup of marinara sauce over the stuffed shells.
• Sprinkle the top with the remaining 1/2 cup of shredded mozzarella cheese.

Bake:

• Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
• Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly.

Serve:

• Remove from the oven and let it rest for 5 minutes.
• Garnish with chopped fresh basil or parsley before serving.