Cut the spring onions into rings, keeping the white and green parts separate. 'Heat 2 tablespoons of oil in a soup pot and sauté the white parts of the spring onions.
Add the potato, stir-fry for 2 minutes, then add the crème fraîche and thyme. Pour in the stock, bring everything to the boil and let the soup simmer gently for 10 minutes.
Meanwhile heat the rest of the oil in a frying pan and fry the mushrooms for 5 minutes, then generously season to taste and drizzle with truffle oil.
Purée the soup with the hand blender and season to taste.
Serve the soup in deep plates, add the fried mushrooms and sprinkle with cheese and the spring onion greens.
