Peal and finely chop the onions & garlic. Peal (or just scrub) the carrots and roughly chop. Peal & roughly chop the celeriac.
Puff the oil into the pan and put over a medium heat. Add the onions & garlic and stir to soften for 2-3 minutes.
Stir in the carrots and celeriac for another minute or so then add the lentils and stock.
Mix everything well together then bring to the boil and bring up to pressure.
Cook for 8-10 minutes (depending on your pressure cooker) then allow to cool by itself.
Remove lid and use hand held blender to create a potage - or add a little more water if you prefer a thinner soup. Season to taste and serve.