Heat a large pan, add 25g butter, onion, and bacon, and fry for two minutes.
Add garlic, leek, celery, and thyme, and fry for one minute.
Add chopped celeriac and stock, bring to a boil, then simmer for 10-15 minutes until tender.
Blend the soup to a fine purée and return it to the pan.
Whisk in the double cream and the remaining 50g of butter.
Season with salt and white pepper.
Fry pancetta until crisp and set aside.
Heat half the remaining butter and olive oil, then stir-fry julienned celeriac for 2 minutes.
Fry bread cubes in the remaining butter until golden-brown and season with salt and pepper.
Ladle soup over julienned celeriac and top with pancetta, croutons, olive oil, and cream.