Add the potatoes to a large pan of cold salted water. Bring to the boil and boil until just tender (about 12-15 minutes)
Drain and arrange on a wire rack to steam for 10 minutes, this will remove excess moisture and prevent gloopy mash.
Meanwhile add the butter, cream and mustard to the warm pan and allow to melt.
Add the potatoes and parsnips and mash with a fork. Swap to a handblender and blend until smooth.
Fold in the watercress, taste and add salt if needed. Serve straight away.