Add the oil to the instant pot and set to saute.
When the oil is hot, add the onions and fry until softened (around 3 minutes).
Add the garlic, thyme and then the mince. Roughly brown.
Now add the carrots, celery, Worcester sauce, beef stock and tomato puree. Stir everything together.
Season to taste and then add the tall trivet to the pot. Put the peeled potatoes on the trivet.
Put the lid on and set for 12 minutes at high pressure. leave under pressure for another 12 minutes and then do a quick release.
Remove the lid and put the potatoes in a bowl. Mash together with milk, butter and salt/pepper. Add the frozen peas.
Add the potatoes back into the pot and sprinkle the cheese on top.
Put the air fryer lid on and set to broil for 8 minutes.
Remove the lid, serve and enjoy.