Preheat the oven to 350F degrees.
Combine the chicken, stock, potatoes, onions, carrots and seasoning in the Instant Pot and pressure cook on high for 4 minutes.
Natural release for 5 minutes, then use the quick release to release the remaining pressure.
Heat the butter and flour for the roux in a cast iron skillet over medium heat.
Cook for about 3-5 minutes, whisking constantly. Remove from heat and set aside.
Turn the Instant Pot to the sauté setting (low if your Instant Pot allows it).
Heat the roux over medium heat and SLOWLY whisk in the milk, whisking constantly until a paste forms.
Stir the paste into the chicken mixture and bring to a simmer until thickened slightly.
Pour the mixture into an ovenproof baking dish.
Place the puff pastry sheet over top of the baking dish and seal around the edges. Cut 3 slits in the top.
Bake for 12-18 minutes, until the puff pastry is golden brown.