Heat a heavy bottom sauce pot over medium heat and add butter and onions. Lower heat too low and continued to cook onions until translucent (about 10 minutes). Add the sliced button mushrooms and dried porcini mushrooms and cook until they have released their juices. Continue to cook until they are almost dry.
Increase heat to medium and add the white and Marsala wine to the mushrooms to deglaze all of the rich flavors. Continue to cook the mixture until it is reduced to a light syrup.
Add the cream and milk and heat until small bubbles form around the edges of the pot. Use an immersion blender or food processor to puree the mushroom soup to a fine texture. Strain the soup through a fine sieve or strainer if you prefer a finer texture.
Finish the mushroom soup with the Banyuls vinegar , salt and white pepper. Serve hot.