1Thinly slice the mushroom caps, discarding the stalks.
2Melt the butter in a heavy pan and cook the spring onion, garlic and thyme, stirring for 1 minute, or until the garlic is golden. Add the mushroom and salt and white pepper. Cook for 3 to 4 minutes, or until the mushroom just softens. Add flour and cook , stirring for 1 minute.
3Remove from the heat and add the stock, stirring continuously. Return to the heat and bring to a boil, stirring. Reduce the heat and simmer gently for 2 minutes, stirring occasionally.
4Whisk the cream into the soup, then reheat gently, stirring. Do not allow the soup to boil. Season to taste with salt and pepper.