Heat 2 tbs. of the butter in a large pot. Add the onion and leeks. Cook over medium-high heat for several minutes, stirring frequently.
Lower the heat and add the potatoes, 1 tsp. salt and 2 cups of the stock. Cover and reduce heat to low and cook for 10-15 minutes.
Heat the remaining butter and olive oil in a large skillet. Add the mushrooms and sauté over high heat until they release their liquid, stirring frequently. Add ½ tsp. salt and the sherry and continue to cook until liquid has reduced by half.
Add mushroom mixture to the soup and add the remaining stock. Bring to a rolling boil, then reduce heat and simmer slowly, partially covered, until the potatoes are soft, about 25 minutes.
If you prefer a slightly thicker soup, make a slurry of 2 tbs. of rice flour and water and stir it in near the end of the cooking time. (Rice flour lends a smoother and less pasty texture than regular wheat flour.)
Garnish with chopped parsley and serve.