Bring a large pot of salted water to a rolling boil and cook the jumbo shells for 8-10 minutes until al dente. Drain in a colander and set aside to cool for about 5 minutes while you prepare the filling.

In a large mixing bowl, combine the ricotta cheese, half of the mozzarella (1/2 cup), Parmesan, and egg. Mix thoroughly with a spoon or spatula until smooth and well blended.

Heat the olive oil in a skillet over medium heat and add the minced garlic. Cook for 30 seconds until fragrant and lightly golden. Toss in the chopped spinach and cook for 2-3 minutes until completely wilted. If using fresh spinach, let it cool slightly and squeeze out any excess moisture with paper towels.

Add the cooled spinach mixture to your cheese blend and stir well to distribute evenly. Season with Italian seasoning, salt, and pepper. Taste and adjust seasonings.

Preheat oven to 375°F (190°C). Spread 1 cup of marinara sauce evenly across the bottom of a 9x13-inch baking dish.

Take each cooled shell and fill with about 2 tablespoons of the ricotta-spinach mixture using a spoon. Arrange them in a single layer in the baking dish with the open side facing up, making sure shells aren't touching.

Spoon the remaining marinara sauce over and around the stuffed shells. Sprinkle the reserved 1/2 cup of mozzarella evenly over the top.

Cover the baking dish tightly with aluminum foil and bake for 20 minutes. Remove the foil and continue baking for another 10 minutes until the cheese is melted, bubbly, and turning golden brown at the edges.

Let the dish rest for 5 minutes before serving. Garnish with torn fresh basil if desired.