Begin by bringing a large pot of salted water to a vigorous boil. Carefully add the jumbo pasta shells and cook them for about 8 minutes until they reach an al dente texture. Once cooked, promptly drain the shells in a colander and lay them out on a baking sheet to cool, ensuring they don’t stick together.

In a large mixing bowl, combine the ricotta cheese, 1 cup of mozzarella cheese (reserving the remaining for topping), Parmesan cheese, chopped spinach, the large egg, minced garlic, Italian seasoning, salt, and black pepper. Mix this delightful filling until it's smooth and creamy, and set it aside.

Using a spoon or piping bag, gently fill each cooled jumbo pasta shell with the prepared cheese mixture, ensuring each shell is generously filled without overstuffing.

Spread half of the marinara sauce evenly on the bottom of a baking dish. Carefully arrange the stuffed shells on top of the sauce in a single layer.

Spoon the remaining marinara sauce over the arranged shells, covering them well. Then, sprinkle the remaining mozzarella and extra Parmesan cheese on top.

Cover the baking dish tightly with aluminum foil and place it in a preheated oven set to 375°F. Bake for 25 minutes, then remove the foil and continue baking for an additional 10 minutes until the cheese is bubbly and golden brown.

After removing the dish from the oven, let the Stuffed Shells Pasta cool slightly. Before serving, garnish with fresh parsley.