Cut the beef into long, thin strips, cutting across the grain.
Heat the oil in a wok or large frying pan over a high heat. Add the garlic and stir-fry for 1 minute. Add the beef and stir-fry for 2–3 minutes until lightly coloured. Stir in the lemon grass and ginger and remove the pan from the heat. Remove the beef from the pan and set aside.
Add the peppers and onion to the pan and stir-fry for 2–3 minutes until the onions are just turning golden brown and are slightly softened.
Return the beef to the pan, stir in the lime juice and season to taste with pepper. Serve with boiled noodles or rice, if liked.