Preheat oven to 350 degrees F. Grease a 9x13 inch pan.
Fill a large pot with water and add 1 tablespoon of salt. Place on stove and bring to a boil.
Peel potatoes and cut into large pieces. Once the water is boiling carefully add the potatoes to the water. Boil for 10-15 minutes until potatoes are softened all the way through. Carefully pour into a strainer.
Add the potatoes back to the pot along with the butter. Use a potato masher, fork, or a handheld mixer, to mash the potatoes.
Add shredded chicken and gravy to the potatoes and mix everything together.
Add the frozen mixed vegetables and gently fold them in.
Spread the mixture into the greased pan.
Make the stuffing according to package instructions.
Spoon the stuffing onto the top of the chicken and then bake for 30-35 minutes .