Cook the jumbo shells in a large pot of salted boiling water according to package instructions until al dente. Drain and set aside to cool slightly.
In a mixing bowl, combine ricotta cheese, chopped spinach, half of the mozzarella cheese, Parmesan cheese, beaten egg, minced garlic, oregano, salt, and pepper. Mix until well combined.
Preheat your oven to 375°F (190°C). Using a teaspoon or a piping bag, fill each cooked shell with the ricotta-spinach mixture.
Spread 1 cup of marinara sauce at the bottom of a 9x13 inch baking dish. Place the stuffed shells in a single layer over the sauce. Pour the remaining marinara sauce over the shells and sprinkle with the remaining mozzarella cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Let the dish cool for a few minutes before serving. Garnish with fresh basil leaves for added flavor and color.
Notes
You can add more vegetables or meat to the filling for extra flavor.