Preheat the oven to 350°F (175°C) and spread the jar of pasta sauce evenly across the bottom of a 13x9-inch baking dish.
Cook the jumbo pasta shells in a large pot of salted boiling water according to package directions until al dente, then drain in a colander and keep warm.
In a medium bowl, combine the ricotta, 3/4 cup shredded mozzarella, egg, chopped spinach, minced garlic, Italian seasoning, salt, and black pepper until evenly mixed.
Spoon or pipe the ricotta mixture into each cooked shell, filling them evenly, and arrange the filled shells in the prepared baking dish on top of the sauce.
Sprinkle the remaining 1/2 cup shredded mozzarella over the filled shells.
Bake uncovered for 20 to 25 minutes, until the cheese is melted and the sauce is bubbling around the edges.
Remove from the oven and let the dish rest at room temperature for 10 minutes, then garnish with chopped Italian parsley and serve.
