Preheat oven to 160˚C.
Combine flour, salt, egg whites and water, mixing into a paste.
Cover a baking tray with baking parchment and spread some salt baking paste on the base of your tray and position the trimmed celeriac on top.
Cover with the celeriac with the remaining salt crust, making sure there are no gaps or holes.
Bake for 3-4 hours until a golden crust has formed.
When cooked, heat butter in a pan and cook until it is starting to colour and foam - this is a beurre noisette. Crack open the celeriac crust and pour in butter. Top with truffle shavings if using. This can be eaten as a side dish.
Alternatively, scoop the baked celeriac out of the salt case and combine in a pan with some softened apple, vegetable stock and double cream. When heated, this can be blitzed into a smooth, creamy soup. Top the soup with shaved truffle, a drizzle of olive oil and a sprinkle of Cornish Sea Salt. If you feel very adventurous, try a confit egg yolk as a luxurious extra topping. Serve with some chunky white bread or toasted caper focaccia.
Try this: if your celeriac had leaves, try blending them with some olive oil to make a vibrant green celeriac leaf oil - delicious on this soup or drizzled on a salad.