Add the olive oil to the instant pot and set to saute.
Roughly chop the onions, carrots and celery and add to the pot.
Season with salt and pepper and fry for around 10 minutes.
Add the rosemary leaves and then the flour. Fry for a further 2 minutes.
Add the stout and stir well. Bring to the boil to evaporate the alcohol.
Add the mustard, Worcester sauce, tinned tomatoes and then the stewing beef. Stir together.
Put the lid on and set on high pressure for 20 minutes. Leave under pressure for a further 10 minutes and then perform a quick release.
Peel and slice the potatoes into approximately 1cm (⅓ inch) slices. Parboil in boiling salted water for 5 minutes. Drain and set aside.
Set the oven to preheat at 180C (350F).
Remove the instant pot lid and put the stew into an oven proof dish.
Top with the sliced potatoes and drizzle a little olive oil or melted butter on top. Add a few rosemary leaves and put in the preheated oven until the potatoes are golden brown - around 40 minutes.
Remove from the oven, serve and enjoy.