Cook jumbo pasta shells according to package instructions. Boil in salted water until al dente, typically 8 to 10 minutes. Drain shells and set aside to cool slightly.

In large mixing bowl, combine ricotta cheese, half cup shredded mozzarella, grated Parmesan, and egg. Mix until well incorporated.

Heat tablespoon olive oil in skillet over medium heat. Add minced garlic and cook for 30 seconds until fragrant. Toss in chopped spinach and cook until wilted, about 2 to 3 minutes.

Add cooked spinach and garlic to cheese mixture. Stir well to combine and season filling with Italian seasoning, salt, and freshly ground black pepper.

Preheat oven to 375 degrees Fahrenheit. Spread 1 cup marinara sauce evenly over bottom of 9x13 inch baking dish.

Take each cooked shell and fill with about 2 tablespoons of ricotta-spinach mixture. Arrange stuffed shells in single layer in baking dish, open side up.

Once all shells are stuffed, spoon remaining marinara sauce over top. Sprinkle remaining half cup mozzarella cheese over shells.

Cover baking dish with aluminum foil and bake for 20 minutes. Remove foil and bake for additional 10 minutes until cheese is melted and bubbly. Let dish rest for a few minutes before serving. Garnish with fresh basil leaves if desired.