In a large heavy-based pot, melt butter over medium-low heat. Add in celeriac, potato, leek, garlic, and onion and season with some salt and pepper.
Sweat vegetables over low heat until they start to soften (10-15 minutes) and add in stock.
Bring soup to a boil and turn heat down and let simmer for another 20-25 minutes until celeriac is tender.
Transfer soup to a liquidiser (we used our Spectablend) or use a stick blender. Blend until soup is smooth and there are no chunks left.
Transfer soup back into your pot and season with more salt & pepper if required. Reheat soup before serving.
When serving, garnish with spring onion, leek ash and freshly shaved truffles. You can shave as much or as little truffle as you wish.