Cut beef in thin, bite-sized slices across the grain. Combine garlic, salt and ginger and rub into beef, mixing well. Set aside for 10 minutes. Mix cornstarch (or barley flour) with water, soy sauce and sesame oil.
Heat a wok, add peanut oil and swirl to coat. Add beef and stir-fry over high heat until colour changes. Add spring onions and fry for 1 minute longer. Add cornstarch mixture and stir until it boils and thickens.