Debone and remove the skin from the chicken thighs. Place the ready thighs in an oven safe 9”x13” pan coated in non-stick spray or lined with parchment paper. Season the thighs with salt and pepper and pour 1/4 c. water in with the chicken thighs. Cover with aluminum foil and bake at 400℉/200℃/Gas Mark 6 for 45 minutes.
Once cooked thoroughly, remove the thighs from the oven and put into a large bowl. Use two forks and in opposite motions to each other hand shred the chicken thighs. Cover and set aside.
Coat a deep-frying pan with non-stick spray or pour olive oil into the pan. On medium heat add your onions and fry for about 2 minutes. Add the garlic, celery, and carrots to the frying pan and fry for about 8 minutes. Once the vegetables have softened, pour them into the bowl with the shredded chicken.
Return your frying pan to the heat and place your butter into the pan and melt. Once the butter has melted, stir in the flour until a paste forms and cook until it turns golden brown. Slowly begin to whisk in the milk. A little bit at a time until all of the milk has been incorporated into the flour and butter and a creamy sauce is formed.
Pour in the 1 c. of chicken broth. Bring the sauce to a gentle simmer. Once it has reached a gentle simmer, add the vegetables and chicken back into the sauce.
Add the Dijon mustard, thyme, and other spices. Cover with a lid and let simmer on a low heat for 15-20 minutes. Stirring occasionally.
While your chicken filling is simmering, roll out your pie dough and place into your pie dish.
Preheat the oven to 400℉/200℃/Gas Mark 6.
Spoon the chicken filling into the pie dish. Brush some of the beaten egg onto the edges of the pie dough and place the pie dough lid onto the top of the pie and crimp the edges together. Use either a spoon or kitchen scissors to remove the excess pie dough. Brush the entire pie dough lid with the beaten egg. Poke some holes into the top of the chicken pot pie. Add your decorative cut outs here if you have any and brush with the beaten egg.
Bake the pie for 35-40 minutes until it is golden-brown and the edges are bubbling.