Heat oil in a large saucepan over medium heat, add onion and garlic and stir occasionally until just tender (5 minutes), then add bacon and stir occasionally until bacon is tender (5-10 minutes). Add celeriac, potato and stock and bring to the simmer, then cover and cook until potato and celeriac are tender (30 minutes). Add crème fraîche and cream and blend, in batches in a blender or with a hand-held blender, until smooth. Season to taste and keep warm.
Meanwhile, for fried chestnuts, bacon and rosemary, heat oil in a non-stick frying pan over high heat, add chestnuts and bacon and stir occasionally until golden (5-6 minutes). Add rosemary, stir until just crisp (1 minute). Serve scattered over soup, with crusty bread.
This recipe is from the June 2012 issue of Australian Gourmet Traveller.