Tip: I suggest reading through all the instructions before beginning.
Set the sheets of frozen puff pastry out to thaw on a floured board.
Cut carrot, potato and celery into small bite sized pieces. Place in a tall microwave safe bowl and add enough hot water to cover by about 2". Microwave on high for about 15-18 minutes or until the veggies are fork tender. Drain when cooked and set aside.
Meanwhile prepare the rest of the ingredients: Cut the mushrooms into eights. Set aside.
Peel the onion and dice it. Set aside.
Mince the parsley and set aside.
Wash the chicken. Trim any large chunks of fat off and discard. Cut into bite sized pieces. A kitchen scissors works well for this.
In a 10 cup capacity skillet (preferably cast iron) cook the chicken with some oil on medium-high, stirring frequently until the pieces are cooked through. Set aside in a bowl.
Preheat the oven to 400 F.
Add the butter, onions and pinch of salt to the skillet and cook on medium-high for about 3 minutes until golden, stirring frequently and adding oil as needed. Turn the heat down to medium. Add the mushrooms and cook an additional 3 minutes, stirring often.
Add the drained veggie mixture and the flour. Cook for 1 minute stirring continuously.
Stir in chicken broth, parsley, chicken, black pepper, white pepper and chicken bouillon. Bring to a boil stirring continuously and continue to cook until mixture has thickened slightly.
When the mixture has thickened, turn off the heat and stir in the cream. Taste and adjust salt and pepper as desired.
Use a mason jar to cut out rounds of puff pastry: Place the jar upside down on the pastry and press down firmly. Move the jar back and forth rapidly until a circle of pastry is cut out. Repeat until you have 11 circles.
Use a sharp knife to make an air slit in the pastry circles.
Use a measuring cup to scoop 2/3 cup of filling into each jar. Wipe any drips off the rims of the jars with a wet cloth. Top each filled jar with a circle of pastry. Brush the pastry with melted butter
Place jars in a large casserole dish (my big rectangular Pyrex is perfect) and fill the dish with water about 1" deep. This is the correct way to bake in mason jars.
Bake in a preheated oven for 25-30 minutes until the pastry is slightly brown and the filling is bubbly.