Sauté the mushrooms in the butter, stirring them occasionally, until they are quite dry, but NOT burnt. Add the remaining ingredients EXCEPT the cream.
Allow the mixture to gently simmer until 50% of the liquid has evapourated. For a very intense flavour you can reduce the liquid until it is almost dry.
Remove the thyme twigs. At this stage you can use a stick blender and break the sauce down slightly, if you prefer a textured, chunky sauce. If you want a smooth, creamy sauce you can liquidise it further.
Now place the sauce on the heat and gradually add the cream, stirring and tasting until you achieve the level of creaminess and flavour balance that you enjoy. Bear in mind that the addition of the sauce to some foods such as pasta may ‘dilute the flavour’. Bolder meats such as grilled steak need a powerfully flavoured sauce, where a more delicate sauce works with chicken breast or fish.
Serve the sauce with meat, poultry fish or pasta.