Preheat oven to 375°F. Grease a large baking dish (I used a 2QT dish) with cooking spray.
Cook and drain the pasta shells as instructed on the box.
Add avocado to a blender or food processor and puree until smooth. Scoop the avocado into a large bowl with the chicken, pepper, salt, pesto, red pepper flakes, olive oil and 1/2 cup mozzarella cheese. Stir to combine.
Combine all of the ingredients for the ricotta mix in a medium bowl. Stir to combine well. Set aside.
In a medium saucepan over medium heat, melt butter for the sauce. Whisk in flour and cook until golden brown, approximately 30 seconds to 1 minute. Whisk in milk, salt, pepper and parmesan cheese. Bring sauce to a simmer until thickened, approximately 4 minutes.
Pour sauce into greased baking dish.
Stuff each pasta shell halfway with the ricotta mixture. Fill each shell the rest of the way with the pesto chicken mixture. Place shells in the sauce in the baking dish.
Top with 1/2 cup shredded mozzarella. Bake for 20 minutes until the cheese is golden brown and bubbly. Serve immediately.