Add 2 tbsp of the butter to the pot and set to saute.
When the butter has melted, add the onion, celery and carrot. Fry until softened (around 3 minutes).
Season to taste and add the potatoes, stock and chicken. Stir together and put the lid on the instant pot.
Set the pot on high pressure for 5 minutes and when the time is up, leave under pressure for a further 10 minutes.
With a couple of minutes to go, prepare the roux. Melt 2 tbsp of butter in a pan and add the flour. Whisk together and slowly add the milk whilst continuing to whisk (this should take around 2 minutes).
Do a quick release and remove the instant pot lid. Add the frozen peas followed by the pre-prepared roux. Stir well together until the sauce is nice and thick (add more milk if necessary).
Place the puff pastry on top of the pie and then put the air fryer lid on. Set to broil for 10 minutes. Remove the lid and check to see if the pastry is cooked.
Serve and enjoy.
Video
Notes
Don't worry if you haven't got an air fryer lid. Just remove the pot contents to an ovenproof dish and put the pastry on this. Cook conventionally in the oven.