Cut the potatoes into ½–¾-inch (12 mm–2 cm) pieces and steam over boiling salted water until tender, about 15 minutes.
Transfer to a bowl, reserving about ½ cup (4 fl oz/125 ml) of the steaming liquid. Using a fork, mash the potatoes coarsely, thinning them to the desired consistency with the reserved steaming liquid. Stir in the butter or olive oil and herbs,