Melt two thirds of the butter and sweat the onion and the garlic in it. Wipe the mushrooms with a damp cloth and slice. Add to the pan and stir until coated with butter. Cook over moderate heat until the mushrooms have collapsed and given up their liquid. Pour in the stock, cover and simmer for 10–15 minutes.
Purée the contents of the pan in a blender or processor but leave a slight texture to the soup. Melt the remaining butter in the pan and stir in the flour for 1–2 minutes. Gradually add the milk, stirring all the time, then add the purée. Simmer for 5 minutes.
Whisk in the crème fraîche and season to taste. Serve in hot bowls; 4–6 servings.
© 1987 Geraldene Holt. All rights reserved.