Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Cook the jumbo pasta shells according to package directions until al dente. Drain, rinse with cold water to prevent sticking, and set aside.
Step 3: In a large skillet, heat olive oil over medium heat. Add the chopped onion and cook until softened, about 5 minutes. Add the minced garlic and cook for another minute until fragrant.
Step 4: Add the thawed and squeezed dry spinach to the skillet. Cook for 2-3 minutes, stirring, to remove any remaining moisture.
Step 5: In a large bowl, combine the ricotta cheese, egg, 1/2 cup Parmesan cheese, parsley, salt, pepper, and the cooked spinach and onion mixture. Mix well until everything is evenly combined.
Step 6: Spread about 1 cup of marinara sauce on the bottom of the prepared baking dish.
Step 7: Carefully fill each cooked pasta shell with a generous amount of the ricotta-spinach mixture. Arrange the filled shells in a single layer over the marinara sauce in the baking dish.
Step 8: Pour the remaining marinara sauce evenly over the stuffed shells. Sprinkle generously with the shredded mozzarella cheese and a little extra Parmesan.
Step 9: Cover the baking dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and lightly golden.
Step 10: Let the Spinach and Ricotta Stuffed Shells rest for 5-10 minutes before serving. This allows the dish to set and makes it easier to serve.