Begin by boiling a large pot of salted water. Add the jumbo shells and cook according to the package instructions until they are al dente. Drain the shells and set them aside to cool slightly.
In a skillet, heat the olive oil over medium heat. Add the minced garlic and sauté for about 1 minute until fragrant. Then add the fresh spinach and cook until wilted, about 3–4 minutes. Remove from heat and allow to cool slightly.
In a mixing bowl, combine the ricotta cheese, shredded parmesan cheese, egg, cooked spinach and garlic mixture, salt, pepper, parsley, basil, and oregano. Mix until well combined.
Preheat your oven to 375°F (190°C). Spread 1 cup of marinara sauce evenly in the bottom of a baking dish. Using a spoon, fill each cooked jumbo shell with the ricotta and spinach filling, and place them in the baking dish.
Once all the shells are filled and arranged in the baking dish, pour the remaining marinara sauce over the top. Sprinkle shredded mozzarella cheese generously over the sauce.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Then, remove the foil and bake for an additional 10–15 minutes, or until the cheese is bubbly and golden.
Remove the baking dish from the oven and let it cool for a few minutes. Garnish with fresh parsley if desired, then serve warm and enjoy your Ricotta and Spinach Stuffed Shells!