Wash the mushrooms and peel them. Peel the onion. Put the mushrooms and onion through the mincer or chop finely. Melt half the butter in a large saucepan and add the mushroom mixture. Cover with a lid and cook gently for about 5 minutes until the juices from the mushrooms run and the onion is soft. Add the stock and the bay leaf. Bring up to the boil and simmer for a further 15–30 minutes. Draw off the heat and pass the liquid and vegetables through a sieve or purée in a blender.
Melt the remaining butter in the hot saucepan and stir in the flour. Cook gently for 1 minute. Add the milk gradually stirring all the time to get a smooth thin sauce. Bring up to the boil and simmer for 2–3 minutes, then season with salt and freshly milled pepper. Stir in the mushroom purée and blend well. The soup should have a fairly thin consistency. Bring back up to the boil, then draw off the heat. Stir in the cream, check seasoning and serve.