Put ceps in a bowl and cover with lukewarm water. Leave to soak for 30 minutes. Heat oil and butter in a large saucepan, add shallots and garlic and cook, stirring occasionally, for 5 minutes, until soft. Add the chestnut mushrooms and cook, stirring occasionally, for 5 minutes. Drain ceps, reserving soaking liquid. Add ceps, stock and lemon rind to pan. Bring to the boil, cover and simmer for 30-40 minutes, until ceps and tender.
Meanwhile, make croutons. Cut bread into small cubes. Heat oil in a frying pan, add bread and fry, turning, until golden and crisp. Remove with a slotted spoon and drain on kitchen paper. Puree soup in a blender or food processor. Return soup to rinsed out pan and stir in enough reserved soaking liquid to give desired consistency. Add tarragon, cream and salt and pepper and reheat gently without boiling. Garnish with croutons and tarragon sprigs and serve.