Bring a large pot of salted water to a boil. Add the 20 large pasta shells and cook according to package instructions until al dente—firm but tender. Drain carefully and rinse under cold water to stop the cooking process and prevent sticking. Set aside to cool.
In a large mixing bowl, combine shredded cooked chicken, thawed and drained spinach, chopped artichoke hearts, ricotta cheese, 3/4 cup of shredded mozzarella, parmesan cheese, garlic powder, onion powder, salt, and pepper. Mix thoroughly until all ingredients are evenly incorporated and the filling is creamy and cohesive.
Take each pasta shell and spoon a generous amount of the filling inside. Place the stuffed shells seam-side up in your baking dish, arranging them snugly but not overcrowded.
Pour the marinara sauce evenly over the top of the stuffed shells. Sprinkle the remaining mozzarella cheese on top. Drizzle olive oil over everything to help with browning and flavor.
Preheat your oven to 375°F (190°C). Cover the baking dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and golden brown.
Once out of the oven, let the dish cool slightly. Sprinkle with fresh chopped basil if desired. Serve warm and enjoy the creamy, cheesy, and flavorful delight.