Step 1Melt the butter in a heavy pot over medium heat. Add the leeks, season with salt and pepper and cook, stirring, until soft, 8 minutes. Add the stock and potatoes and bring to a boil. Reduce the heat and simmer, uncovered, until the potatoes are tender, 25 to 30 minutes. Stir in the watercress and cook until wilted but bright green, 2 to 3 minutes.
Step 2Carefully transfer to a blender or food processor in batches and puree until smooth. Strain through a coarse sieve into a clean saucepan and bring to a gentle simmer.
Step 3Stir in half-and-half and cheese until the cheese is melted. Adjust the seasoning. Garnish with sprigs of watercress and pecans.