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, if using, and season to taste with salt and pepper. Serve right away.