Cook the potatoes in a large pan of salted water for 15 minutes or until tender. Drain well, then return to the pan to steam-dry for a few mins before mashing.
Push the potatoes to one side and add the milk and butter to the other side of the pan. Heat gently until the butter melts, then stir into the potatoes with the watercress and some seasoning.
650g floury potato, cut into chunks
100ml milk (1/2 cup) (substitute unsweetened oat, coconut, or other non-dairy milk of choice to make this recipe vegan)
25mg butter (substitute vegan butter or a high quality oil like olive or coconut oil to make vegan)