6 Tbs butter separated
1/2 onion diced  (you are going to strain this out later so no points for neatness)
3 Tbs flour
About 1 1/2 quarts chicken stock (preferably homemade, but store bought will work, be careful with salt)
2 pounds mushrooms stemmed, caps sliced thin (I used cremini  and a few oyster but pick your favorite, keep stems separate from caps)
1/4 cup Marsala wine
2 egg yolks
1/2 cup cream
salt and pepper to taste

Over medium low heat cook onions in 3 Tbs butter until soft (less than 10 minutes).  Meanwhile bring stock to boil.  After onions are softened, stir in flour and cook for 3 - 4 minutes.  Off heat whisk boiling stock into onions, add mushroom stems and cook on a low simmer for about 20 minutes.

In a separate pan, heat remaining 3 Tbs butter over medium heat until it foams and then the foam subsides.  Add sliced mushroom caps and Marsala wine (this is my addition)  and season the mushrooms.  Put a lid on the pan and cook until soft, about 8 - 10 minutes.

Strain soup-base of stock, onion and mushroom caps. pressing to get all the juices. 

In a large mixing bowl whisk together the egg yolks and cream.  Slowly, by spoonfuls, whisk in hot soup base.  Don't do this too fast or  your eggs will scramble instead of thickening the soup.  Once you have about half the mixture you should be fine to return it all to the saucepan.  Then add the mushroom caps and any liquids released by cooking to the soup base.  Return to medium low heat and cook for a few minutes to thicken.  Do not let it come to a simmer, you could still get scrambled eggs and to go through that at this point would be heartbreaking.