Sauté the onion and leek in the butter over low heat for five minutes or until translucent but not browned.
Add the mushrooms and sauté until all the liquid has evaporated. Do not allow the mushrooms to brown.
Bring the stock and cream to the boil in a saucepan. Add the potatoes.
Add the mushroom mixture and cook for 10 minutes over medium heat.
Remove from the heat and transfer to the bowl of a food processor.
Blend to form a puré and press through a sieve. (Add more stock if the soup is too thick.)
Season with salt and pepper and heat the soup just before serving. Whisk for two minutes until foaming to obtain a cappuccino effect.
Serve with cheese and garlic crostini.
CHEESE AND GARLIC CROSTINI
Slice a French loaf into diagonal slices and rub with a clove of garlic.
Sprinkle with Parmesan cheese and place under the oven grill until the cheese begins to melt.