Prepare the leeks:  wash the whole leek, then as you slice it, throw the slices into a big bowl of cold water.  There's often more dirt inside the first few layers of leek, and this will get it out.  Lift the clean pieces out and put on a cloth or paper towel to soak up excess water.

Ingredients

• 2 tablespoons (1/4 stick) butter, or olive oil, or 1 tbsp of each
• 3 cups sliced leeks (white and pale green parts only)
• [optional:  sliced fennel bulb]
• 6 cups water or broth
• 2 pounds potatoes, cut into 1/2-inch pieces

Preparation

1. Heat oil/butter in heavy large pot over medium-high heat.
2. Add leeks [and fennel if using]  and sauté until leeks are translucent, about 7 minutes.
3. Add broth and potatoes and bring to boil. Reduce heat to medium-low.
4. Simmer soup until potatoes are very tender, about 15-20 minutes.
5. Working in batches, purée soup in blender (if you have a hand blender, you can stick it right into the pot). Return to same pot & stir.   Season with salt and pepper & serve.