1. For the chantilly, place the creams and bacon in a stainless steel bowl, cover with plastic wrap and refrigerate overnight.
2. For the veloute stock, peel the celeriac and place the skin in a saucepan. Finely chop the flesh and add half to the pan. Reserve the remainder for the soup. Add the bay leaf to the saucepan and half of the onion, garlic and thyme. Cover with water (about 1 litre), bring to the boil and simmer gently for 30 minutes. Pass through a fine sieve, discarding the solids, and reserve. This should yield 700ml stock.
3. To make the soup, melt the butter in a heavy-based saucepan over a medium-high heat, and allow to foam lightly without colouring. Add the remaining onion and garlic, reduce the heat and simmer gently for 5 minutes without colouring until translucent. Add the remaining drained celeriac and chestnuts and season to taste. Cook gently for a further 5 minutes until soft and aromatic, but still without colour. Add the reserved stock and remaining thyme and bring to the boil. Reduce the heat and simmer gently for 20 minutes. Add the milk and remove from the heat. Strain, reserving the liquid. Put the solids in a blender and process until smooth. Slowly add the reserved liquid; this will emulsify the soup, making it smooth and velvety. Season to taste with salt, pepper, a little lemon juice and a splash of Madeira, then pass through a-fine sieve and keep hot.
4. Meanwhile, remove the cream and bacon from the refrigerator and discard the bacon. Whip the cream to stiff peaks and season to taste with salt, pepper, and a little lemon juice.
5. To serve, preheat the oven to 180°C. Lay the pancetta between 2 non-stick baking sheets and bake until golden and crisp, about 10 minutes. Drain on paper towels and break each slice into 3 pieces.
6. Place the oil in a deep saucepan over a moderately high heat and bring to 190°C, or until hot but not smoking. Add the celery leaves, a few at a time, and fry until crisp, 8-10 seconds. A good sign is that the oil will stop splattering when the moisture in the leaves has dispersed. Quickly remove with a slotted spoon and drain on paper towels. Ladle the hot soup into warm serving bowls and add a spoonful of the chantilly, forming into a quenelle shape if desired. Scatter the pancetta pieces and celery leaves around and serve immediately.
Recipe by Justin North (Becasse Restaurant, Sydney), published in Vogue Entertaining & Travel April/May 2005