Hit the "saute" button and melt the butter, then add the onion and cook for a few minutes until transparent.
Add the potatoes, celeriac, mushrooms and coriander and saute for another few minutes until aromatic
Add the chicken broth and close and lock the lid
Hit "cancel" to reset the Instant Pot, then press "soup"
When it's done (30 minutes once pressure is reached) manually release the pressure.
Puree with an immersion blender (or transfer in batches to a blender) until completely smooth
Add salt to taste and a few twists of freshly ground pepper
Ladle into shallow bowls, garnish with chopped parsley and a small drizzle of truffle oil.
You can also do this on the stove in a pot. Simply sauté the onion in butter, follow the instructions up until locking the lid of the pressure cooker and instead, put a lid on the pot and simmer for one hour. Continue following the directions to puree the soup, season, garnish and serve.
Recipe by Sustainable Dish at https://sustainabledish.com/creamy-celeriac-soup-truffle-oil-instant-pot-stovetop/