Set the Instant Pot to "Saute," melt the butter in the bottom of the pot, and then add in the onion, carrot, and celeriac with a big pinch of sea salt. Saute the veggies for about 10 minutes stirring occasionally while you chop up the rest of the veggies.
Turn off the Saute feature and toss in the rest of the veggies and the bone broth.
Put the lid on, close the valve, and press "Soup". Bring the time down to 20 minutes, and it will turn on automatically. Once the pot comes to pressure, the 20 minutes will count down.
After the 20 minutes of pressure cooking is done, turn the pot off, and open the valve to let the pressure out. Puree the soup with a handheld blender or in batches in a regular blender. After the soup is pureed you can sea salt and pepper to your taste. Garnish each bowl with parsley if you wish.