I can't tell you the number of times I've seen young cooks throw away perfectly good parts of a leek. You pay as much for the green part as you do for the white part, so you might as well use the whole thing!
You want this soup to have a light green hue to it, so only discard the toughest outer leaves of the leeks. Before you get started, be sure to clean the leeks thoroughly, as dirt and sand can get caught in between the many layers. (Slice off the roots and cut the leek lengthwise. Use warm water to rinse out each layer.)
I'm garnishing this recipe with sautéed chanterelle mushrooms. I'm using rehydrated mushrooms I saved from last year's foraging adventures, but depending where you live, you can probably find chanterelles popping up in the wild right now. Here are tips for foraging for mushrooms safely.