In a medium saucepan, bring the chicken stock to a boil.
Add the chicken breasts, return to a boil, then reduce the heat.
Simmer, covered, for 10 minutes or until the chicken is thoroughly cooked.
Remove the pan from heat and let the chicken stand in the poaching liquid for 10 minutes.
Preheat your 5- or 6-quart air fryer to 350°F for 3 minutes.
Stack the puff pastry sheets on top of each other.
Place a 6-inch x 8¾-inch, 6-cup rectangular dish on the pastry stack.
Cut around it to create a rectangle the same size, then cut four slits crosswise in the center of the pastry.
Alternatively, use another dish of a similar size, ensuring it is no larger than the pastry sheet.
Brush the pastry with beaten egg.
Carefully pull out the air fryer pan and basket.
Spray the basket with cooking spray and place the pastry in the basket.
Slide the pan and basket back into the appliance.
Maintain the temperature at 325°F and set the timer for 15–18 minutes or until the pastry is puffed and beautifully golden.
While the pastry is cooking, remove the chicken from the liquid and chop it coarsely.
Reserve 1 cup of the poaching liquid, keeping the remaining liquid for another use or disposing of it.
Heat the butter in a medium saucepan and cook the leek and celery, stirring until the leek softens.
Add the flour and thyme, and continue stirring for 1 minute.
Gradually stir in the reserved poaching liquid, milk, and cream.
Cook, stirring until the mixture boils and thickens.
Stir in the chopped chicken and mustard, and season to taste.
Spoon the hot chicken mixture into the dish, then carefully place the cooked puff pastry on top.
To serve, you can scatter extra thyme leaves on the pie for garnish, if desired.
