Preheat the oven to 400 degree Fahrenheit. Lightly grease the pie dish with cooking spray or butter & line the u”bottom with the thawed pie crust and set aside. (You can bake the pie dish for 10 minutes, this way it won't be soggy on the bottom in the end, but this is totally optional.)In a skillet over medium heat melt the butter, add the onions and cook until translucent, stirring constantly, about 5 minutes. Add in the flour & cook it while stirring for about 1 minute. Add in the chicken broth and whisk or stir together with the flour mixture until there are no more lumps. Add in the heavy cream & stir. Add all your seasonings and stir until incorporated in the mixture. It should be thicker and creamy at this point. Add in the veggies and chicken. Stir to combine. Put the mixture into the pie dish & smooth it out. Place the puff pastry on top and cut off the excess pastry hangin on the sides. Cut a cross in the center of the pie so the pie can breathe while baking. Whisk together the egg and water. Lightly brush the mixture over the top of the puff pastry. Bake in the oven for 35 minutes or until golden brown. Let stand for 10 minutes when finished. Eat & enjoy!