Preheat the oven to 375°F (190°C) and lightly spray a 9×13-inch baking dish with cooking spray; set aside.
Cook the jumbo pasta shells according to package directions until al dente. Drain and allow shells to cool slightly so they are easy to handle.
In a mixing bowl, combine the chopped cooked chicken, ricotta cheese, pesto, garlic powder, dried basil, and 1 cup of the shredded mozzarella until evenly mixed.
Spoon the chicken and cheese mixture into each cooked shell and arrange the stuffed shells in the prepared baking dish in a single layer.
Pour the jar of spaghetti sauce evenly over the stuffed shells, then sprinkle the remaining 1 cup shredded mozzarella on top.
Bake uncovered for 25–30 minutes, until the sauce is bubbly and the cheese is melted and lightly golden. Let rest a few minutes before serving.
Notes
Use a rotisserie chicken to save time.
Cook shells until just al dente so they hold filling without getting mushy.
You can freeze the assembled dish before baking.
Let the baked dish rest briefly to set before serving.