Bring a large pot of salted water to a boil. Add the 20 jumbo pasta shells and cook according to package instructions until al dente, about 9-11 minutes. Drain and rinse under cold water. Set aside on a clean kitchen towel to dry.
In a large mixing bowl, combine 1 cup ricotta cheese, 1 cup chopped fresh spinach, 1/2 cup grated Parmesan cheese, 1 egg, Italian seasoning, salt, and pepper. Mix well until evenly incorporated.
Using a spoon or small cookie scoop, fill each cooled pasta shell generously with the ricotta and spinach mixture. Arrange the stuffed shells in a single layer in a greased 9x13 inch baking dish.
Pour 2 cups of marinara sauce evenly over the stuffed shells. Sprinkle 1 cup shredded mozzarella cheese on top.
Preheat oven to 375°F (190°C). Cover the baking dish with aluminum foil and bake for 25 minutes. Remove foil and bake an additional 10 minutes until cheese is bubbly and golden.
Remove from oven and let cool for 5 minutes. Garnish with fresh basil leaves. Serve warm and enjoy!