In a large pot of water, boil pasta shells until al dente; according to package directions. Drain the water, then rinse noodles in cold water. Set aside.
In a medium bowl, combine the cooked shredded chicken, 1 to 1 ½ of the containers of chive and onion cream cheese (depends on how wet you want the mixture), 1 cup shredded mozzarella cheese and the basil pesto. Stir mixture gently until combined.
In a 9×13 baking dish, spread about 1 cup of marinara sauce in the bottom. Take one pasta shell and stuff it with 3 Tablespoons of chicken/cheese mixture. Place shell in prepared dish, and then repeat process with remaining shells and chicken/cheese mixture. (There should be enough filling to fill at least 24 shells.)
Pour the remaining marinara sauce evenly over the stuffed shells, then sprinkle the remaining mozzarella cheese all over the top.
Bake, uncovered, for 25 to 30 minutes or until the cheese is melted and bubbly. Remove from the oven and sprinkle with chopped, fresh parsley (optional). Allow to stand for 5 minutes before serving. Enjoy!