4 tablespoons unsalted butter ((1/2 stick))
1 1/2 cups finely sliced leeks ((about 1 large))
2 packages Yukon Gold potatoes (peeled and cut into 1/2-inch dice (4 cups))
7 cups chicken stock
6 ounces (4 cups packed) watercress (tough stems removed)
1 cup half and half
  Salt and freshly ground pepper (to taste)