Place a vegetable steamer in a large saucepan. Add water to come slightly under the bottom of the steamer and bring it to a boil. Place the potatoes in the steamer insert, cover the pan tightly, lower the heat and steam the potatoes until just barely tender, 15 to 20 minutes. A skewer or the tip of a sharp knife inserted into the potato should go in with a little resistance and the potatoes should still be firm. With tongs, remove the potatoes to a bowl of iced water and set them aside until cooled, about 5 minutes. Drain the potatoes well. Peel and set them aside on paper towels or refrigerate until ready to brown them.

When the roast leg of lamb is done, spoon 2 tablespoons of the roasting fat into a large heavy frying pan (preferably cast iron) that will hold the potatoes in one layer. Heat the pan over medium heat until the fat is bubbling hot. Add the potatoes, and sprinkle with the rosemary, lemon juice, salt and pepper. Sauté for 10 to 15 minutes, shaking the pan occasionally to brown the potatoes on all sides, until they are golden brown.