Prepare the marinade by whisking together onion, garlic, soy sauce, ginger and sugar. Add the meat, mix well, cover and marinate in the fridge for 1 hour.
While the meat is marinating, carefully separate lettuce leaves, discarding large outer leaves, wash and pat dry. Prepare the spring onion, carrot and chili.
Preheat chargrill or fry pan to high. Cook beef, reserving liquid, in batches if necessary, until just cooked. When beef is cooked add remaining liquid to the pan and reduce until thickened slightly.
Divide rice among prepared lettuce leaves. Top with beef, scatter with spring onion, carrot, chili and sesame seeds (if using). Drizzle with remaining cooking juices and serve.
