For the Wild Mushroom Garnish:

Fill a medium saucepan three-quarters full of water, add 2 teaspoons salt, and bring to a boil over high heat.

Blanch the green part of the leeks until tender, about 30 seconds. Cool in a bowl of ice water and drain.

In a medium saute pan over medium-high heat, melt the butter. When it begins to bubble, add the mushroom caps and saute until tender.

Season with salt and pepper, add the drained leek greens, and toss to heat through. Drain.

For the Potato Leek Soup:

Place a large soup pot over medium-high heat until hot, about 2 minutes.

Add the bacon and render until golden brown, about 2 minutes. Reduce heat to low, add the butter, and melt.

Raise the heat to medium, add the leeks and mushroom stems, and sweat until softened but not colored, about 8 minutes. Raise the heat to high, bring to a boil, and add the potatoes and stock.

Reduce the heat to medium and simmer until the potatoes are tender, about 20 minutes.

Add the heavy cream, raise the heat to high, and bring to a boil. Reduce the heat slightly and simmer 2 minutes, skimming if necessary.

In a blender or in the pot using a hand-held blender, puree well and season with salt and pepper, then strain through a coarse strainer. If the soup is too thick at this point, bring the extra stock to a boil and add it.

To Serve:

Place a spoonful of the mushroom garnish in the bottom of a large soup plate. Ladle the hot soup over the garnish and serve hot.