Heat the sesame oil in a large skillet or wok over medium heat. Add gingerroot and garlic; cook for 1 to 2 minutes or until soft and fragrant. Raise skillet (or wok) heat to medium-high. Add beef; cook, stirring occasionally, for 4 to 5 minutes or until browned. Stir in broth and fish sauce; bring mixture to a boil then reduce heat to low, partially cover and cook for 25 to 30 minutes or until beef is tender and liquid has thickened.