Cut the beef diagonally into very thin slices. Put into a bowl then rub with sugar and set aside for 10 minutes. Add the garlic, spring onions, soy sauce and sesame oil and set aside for at least 15 minutes.
Heat a large cast iron frying or griddle pan on a high heat until very hot. Brush lightly with oil. Lay the strips of meat in it in a single layer. You may need to do it in batches. Once brown, turn the pieces over and quickly do the other side. Serve immediately.