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