Melt 1/2 the butter in a large sauté pan.
Add onions and sauté for a few minutes.
Add mushrooms and the rest of the butter and cook for 10 minutes until caramelized.
In a large saucepan bring milk, broth, and cream cheese to a simmer over medium heat.
Add mushroom mixture and mix well.
Season with tarragon, salt, and black pepper.
Let simmer for 10 minutes, stirring constantly making sure milk does not scald.
Serve hot and enjoy this comforting soup on a cold winter day!