Melt 2 tbs of butter in a soup pot and add the onion and leeks.
Cook them over high heat for several minutes, stirring frequently, then lower the heat and add the potatoes, 1 tsp of salt, and 2 cups of the stock.
Cover the pot and stew the vegetables over a low heat for about 10 minutes.
Heat the remaining butter and the olive oil in a wide skillet.
Add the mushrooms and sauté them over high heat until they begin to release their juices, stirring as needed.
Add ½ tsp salt and the wine, and cook until the wine is reduced and syrupy.
Scrape the mushrooms into the soup pot and add the remaining 4 cups of stock.
Bring to a boil, then simmer slowly, covered, until the potatoes are completely soft (about 25 minutes).
Taste the soup and season with more salt, if needed, and freshly ground black pepper.
Add the cream, if using, and heat through. If you have a stick blender, you may purée the soup.
Serve with garnish.