Melt the butter in a large pot over medium heat and add the onion. Cook until the onion is translucent, 3 to 5 minutes. Add the fresh mushrooms and cook, stirring occasionally, until the mushrooms are tender, 5 to 7 minutes.
Add the stock and bouquet garni. Bring to a boil, decrease the heat to simmer. Cook until the mushrooms are very soft, about 30 minutes.
Remove the bouquet garni. Purée the soup with an immersion blender.Leave it coarse for a more rustic soup, or purée it until smooth for a more elegant soup. Add the cream and stir to combine. Taste and adjust for seasoning with salt and pepper. Ladle into warmed bowls and serve immediately.