Cut the potatoes into ¾-inch (2-cm) pieces and cook in boiling salted water until tender, about 15 minutes. Drain potatoes, reserving about ½ cup (4 fl oz/125 ml) of the cooking liquid, and return to the hot pot. Using a fork, mash the potatoes coarsely, thinning them to the desired consistency with the reserved cooking liquid. Stir in the butter or olive oil and season