Prep the Pasta Shells: Bring a large pot of salted water to a rolling boil. Add the jumbo pasta shells and cook according to package directions, but aim for al dente. You want them firm enough to handle without tearing. Overcooked shells will make stuffing difficult. Drain the shells thoroughly and rinse with cold water to prevent sticking. Lay them out on a baking sheet or clean towel to cool.

Sauté the Spinach and Garlic: In a large skillet, heat olive oil over medium heat. Add the minced garlic and cook for about 1 minute until fragrant. Add the fresh spinach in batches, stirring until it wilts completely. Once wilted, remove the spinach from the heat. Allow it to cool slightly, then squeeze out as much excess water as possible using a clean kitchen towel or paper towels. Chop the squeezed spinach finely.

Prepare the Ricotta Filling: In a large mixing bowl, combine the ricotta cheese, chopped spinach, 1 cup of shredded mozzarella cheese, grated Parmesan cheese, the egg, dried oregano, red pepper flakes (if using), and chopped fresh parsley. Season generously with salt and black pepper. Mix everything together until well combined.

Preheat Oven and Prep Baking Dish: Preheat your oven to 375°F (190°C). Pour one jar of marinara sauce into the bottom of a 9x13-inch baking dish, spreading it evenly.

Stuff the Shells: Take each cooled pasta shell and carefully fill it with a generous amount of the ricotta-spinach mixture. You can use a small spoon, a piping bag, or even your hands. Place each filled shell seam-side up in the prepared baking dish over the marinara sauce. Arrange them snugly in a single layer. Repeat this process until all the shells are stuffed and arranged.

Top with Sauce and Cheese: Once all your Spinach Ricotta Stuffed Shells are neatly arranged, pour the remaining jar of marinara sauce evenly over the top of the shells. Make sure most of the shells are covered with sauce to prevent them from drying out during baking. Sprinkle the remaining 1 cup of shredded mozzarella cheese over the top of the sauce.

Bake to Perfection: Cover the baking dish tightly with aluminum foil. Bake for 25 minutes. Then, remove the foil and continue baking for another 10-15 minutes, or until the cheese is melted, bubbly, and lightly golden brown. The internal temperature should be hot throughout.

Rest and Serve: Once baked, remove the Spinach Ricotta Stuffed Shells from the oven and let them rest for 5-10 minutes before serving. Garnish with additional fresh parsley or a sprinkle of Parmesan cheese, if desired. Serve these delicious stuffed shells with spinach and ricotta hot and enjoy!