Soak lemon grass in warm water for 2 hours and then chop fine. Heat oil in frying pan over high flame. Add shallots and garlic. Fry for 1 minute. Add lemon grass, then stir and add beef. Break up any large chunks of meat. Turn the heat down to medium and fry until the meat browns. Add shrimp sauce, sesame seeds, sugar, and black pepper. Keep stirring until the meat becomes dry. Remove from pan and serve over rice.