Place the mushrooms in a small bowl and cover with 150ml boiling water. Leave to soak for 15 minutes. Strain through a sieve, reserving the liquid, then roughly chop the mushrooms.
Meanwhile, heat 1 teaspoon of the oil in a pan and cook the onion for 3-4 minutes until softened. Add the chopped sage and chestnuts and cook for a further 3-4 minutes. Add the rice and stir well to coat with the oil.
Pour in the warm stock and bring to the boil. Add the chopped mushrooms and reserved liquid. Season well, then cover and simmer for 20 minutes, stirring occasionally until the rice is tender.
Fry the reserved sage leaves in the remaining oil until crisp, then drain on kitchen paper. Divide the pilaff between 4 bowls and garnish with the sage leaves. Serve drizzled with a little truffle oil and a dollop of crème fraîche.