Peel the celeriac and cut it into large chunks. Place it into the inner pot with enough water just to cover it.
With the pressure cooking cover in place, lock the lid. Select Pressure Cook and set the time to 8 minutes with a quick pressure release.
Unlock the lid and drain water from the celeriac using a colander, then set aside in a bowl. Blend the celeriac into a smooth puree. Add 1 tsp of butter, and 5g of chopped parsley and season to taste with salt and pepper. Set the mash aside.
Rinse your inner pot and place into the pot base. Select Sauté and add a small drizzle of oil, the onion and the leeks.
When soft, add the butter to the inner pot and stir in the flour. Once the roux is smooth, and no streaks of flour remain, stir in the milk and cook until thickened. Season generously with salt and pepper.
Dice the cod into bite-sized chunks, then add this to the creamy leeks, along with the prawns and peas & sweetcorn frozen mix. Stir everything together then flatten the mixture with a spoon.
Top the fish pie filling with the herby celeriac mash and fluff with a fork. Then lightly sprinkle the parmesan over the top.
Remove the pressure cooking cover and lock the lid.
Select Air Fry, and set the temperature to 200°C and cook for 10-12 mins until the mash is golden.
Prawns and cod can be replaced with any suitable fish, from a pre-packaged fish pie fix to any bits of unused fish you have going spare in the freezer. Just be sure to thoroughly defrost all your fish and prawns before cooking.