Set instant pot to saute, add oil, onion, celery, carrots, and garlic to the pot. Cook until softened, stirring occasionally, about 5 minutes. Add meat and cook until browned on all sides, about 3 minutes.
Add seasonings, potatoes, stock, and Worcestershire sauce. Stir to combine.
Assemble pressure lid, making sure the pressure release valve is in the seal position. Select manual high pressure. Set time to 20 minutes.
When pressure cooking is complete, quick release the pressure by moving the pressure release valve to the vent position. Carefully remove lid when unit has finished releasing pressure.
Select saute medium. Let stew come to a simmer, then add peas to pot.
Add flour to the pot and slowly stir into stew. Allow stew to thicken.
Spoon into a pie dish and leave to cool for at least 30 minutes if you have time. Smooth the pastry over the top of the filling. Decorate the pie with cut out shapes if you feel that way inclined and brush with the beaten egg.
When you are ready to bake the pie, preheat the oven to 220°C/425F fan and bake for about 30 minutes until the filling is hot throughout and the pastry is crisp and golden.