Meanwhile, bring a small saucepan of salted water to a boil. Add the potato and cook until crisp-tender, about 5 minutes; drain. Heat 1 tablespoon of the olive oil in a large skillet. Add the onion and cook over moderately high heat for about 5 minutes, stirring occasionally, until softened. Add the lamb and cook, stirring occasionally to break up the meat, until no liquid remains and the lamb is beginning to brown, about 10 minutes. Add the potato and stock and cook over moderate heat until the mixture is dry and the potato is tender, about 8 minutes.