Trim off the tough green portions of leeks. Halve lengthwise and rinse under cold running water to

get rid of grit and sand. Slice finely crosswise.

In a large pot, melt butter. Add leeks and cook until wilted, about 5 minutes.

Add potatoes and stock. Bring to a boil. Season with salt and pepper.

Reduce heat and simmer until potatoes are very tender and falling apart, about 30 minutes. Turn off heat and puree with immersion blender until smooth.

Stir in half and half or cream and top with watercress.

Serve immediately.