Saute onion in butter and olive oil until translucent.
Clean the mushrooms by wiping them with a dry paper towel and add them all except about a cup of the button mushrooms. (you'll add them at the end, sliced)
Add the cayenne, nutmeg, salt, pepper and the thyme.
Add the flour and cook for one minute. The mixture will be thick and gloopy-but not to worry!
Now, add the stock and stir well to incorporate the flour. You can use a whisk if needed to break up the clumps of flour.
Add the rest of the ingredients and let the flavors mix together well. Once you add the milk products, you want to keep the soup from boiling. Cook at mostly low heat.
Remove the thyme stems and serve with crusty bread.
(The reason for adding a cup of the mushrooms at the end is so you'll have different layers of texture in the soup, some mushrooms well cooked and others with a bit more crunch!)