Bring a large pot of salted water to a boil. Add pasta shells, and cook according to package directions until al dente. Drain, and rinse under cold water until cool enough to handle.
Cook chicken in a large skillet over medium-high heat until no longer pink, about 8 minutes. Transfer chicken to a cutting board, and cut into small (diced) pieces.
Preheat oven to 350 degrees F.
In a large bowl, stir together ricotta cheese and sour cream. Add basil, parsley, and nutmeg, stir to combine. Add chicken and half the mozzarella cheese to the ricotta mixture. Mix until well combined.
In a medium bowl, whisk together Alfredo sauce and pesto. Spoon about 3 tablespoons of sauce in the bottom of a 9″ x 13″ baking dish, and spread evenly.
Once pasta is cooled, gently open shells and spoon 2 to 3 tablespoons of the chicken mixture into each shell, until it’s nice and full but still able to close. Place filled shell in the prepared baking dish. Repeat with remaining pasta and chicken mixture.
Top shells with remaining sauce and then sprinkle with remaining mozzarella.
Bake for 25 minutes, until cheese is melted and sauce is bubbly. If you like that golden brown cheese look, you can broil the pasta on HIGH for a few minutes once it’s done baking.
Carefully remove the pasta from the oven and serve warm.