Place the chopped Rooster potatoes, carrots and shallots into a pan of cold water and bring to a very gentle boil.
Cook everything for around 20 minutes, or until tender.
Drain the vegetables, then mash together into a coarse purée. Add the mustard, crème fraîche and butter then season to taste.
Stir in the watercress until it is just wilted but still crisp.
Cook's tip: This dish goes particularly well with pork, so try serving with roast ham, grilled gammon or sausages.