Peel and chop the onion
Heat the oil in your instant pot or stockpot and set the onion to soften (use the saute setting for instant pot)
Stir occasionally while you prepare your other vegetables
Trim and chop the celery and leek. Cut into chunks of about 1-2cm
Peel and chop the potato
Add the leeks and celery to the instant pot and stir to soften on saute for a few minutes
Add the potatoes, stock and a good seasoning of salt and pepper
If you are using an electric pressure cooker, put the lid on and set to cook on high for 30 minutes. If you are cooking this in a regular stockpot, bring to the boil then reduce the heat to a gentle simmer and cook for 40 minutes or until all the vegetables are soft, stirring occasionally
Release the steam from the instant pot
Using a stick blender, liquidise the soup to the texture of your choice. You can remove some of the vegetables with a slotted spoon, then return them after liquidising if you prefer.
Adjust the seasoning, adding salt and pepper to taste
Serve with the garnish of your choice. I like creme fraiche and a few chopped celery leaves