Begin by bringing a large pot of well-salted water to a boil. Once boiling, add the jumbo pasta shells and cook for about 7 minutes, stirring often to prevent sticking, or until they're 3 minutes shy of al dente.

Drain the shells and rinse them under cold water to stop the cooking process. Set aside.

In a mixing bowl, combine the ricotta or cottage cheese, grated Parmesan cheese, egg, steamed spinach, minced garlic, chopped parsley, dried basil, salt, and pepper. Mix well until all the ingredients are thoroughly combined.

Preheat your oven to 375°F (or 400°F for a slightly crispier top). Spread half of the marinara sauce on the bottom of a 9x13 baking dish.

Using a piping bag or a spoon, carefully fill each cooked shell with the cheese mixture and arrange them in a single layer in the baking dish.

Pour the remaining marinara sauce over the filled shells and sprinkle shredded mozzarella cheese on top.

Bake in the preheated oven for 25 minutes, or until the dish is hot and bubbly. If you prefer a faster method, cover the dish with foil and bake for 20 minutes at 425°F, then uncover for an additional 5 minutes to melt the cheese.

For best results, consider preparing the stuffed shells ahead of time. You can assemble them and refrigerate or freeze until you're ready to bake. This allows the flavors to meld beautifully. Additionally, feel free to customize the filling with your favorite herbs or spices, and don't forget to serve with extra marinara sauce on the side for dipping! Frozen spinach is convenient when you need a quick option for your stuffed shells.