Peel and chop the onion. Peel and roughly chop the celeriac. Clean and roughly chop the mushrooms.
Squirt the oil into the pan over a medium heat. Add the onions and stir until softened.
Put in the mushrooms, allow them to release their juices, then add the celeriac and stir well.
Pour in the stock and bring to the boil, stirring a couple of times.
Put on the lid and cook under pressure for 10 minutes. Cool with running water.
Use a hand held blender to create a smooth potage. If not watching the calories it's delicious with crusty fresh bread!