Peal & roughly chop all the vegetables.
Puff the oil into the pan over a medium heat to gently soften the onion and sweat the mushrooms, then add the garlic & leek.
Stir in the parsnip, potato and celeriac and cook over a low heat for a couple of minutes.
Add the stock, put on the lid and bring up to pressure.
Cook for 10 minutes then turn off the heat and cool using cold water as per manufacturer's instructions.
Use a hand-blender to create a smooth soup. Serve garnished with some spice or fresh herbs of your choice.