1. Mix the garlic, ginger with half of the soy sauce and cornstarch and marinate the beef for 30 minutes in the refrigerator.
2. Heat a wok, add the oil and then the beef and its marinade: cook for two minutes, stirring occasionally.
3. Remove the beef and set it aside on a plate, then replace it with the onions and a small glass of water: sauté the onions for 2 or 3 minutes, then add in the following order: fish sauce, the remaining soy sauce, sugar, and lime juice.
4. Cook for 30 seconds, then add the meat back in and cook for another two or three minutes, stirring occasionally.
5. Sprinkle with fresh cilantro and serve with white basmati rice.