In a large bowl, combine soy sauce, rice vinegar, rice wine, lime juice and sesame oil. Stir in onion, basil, mint, lemon grass and crushed peanuts. Season with chilli paste, coriander and garlic salt.
Layer flank steak with marinade. Refrigerate 6 hours or overnight.
Preheat an outdoor barbecue for medium high heat, and lightly oil grate.
Lightly grease a large sheet of aluminium foil. Drain liquid from marinade. Layer meat on foil with the remaining non-liquid marinade ingredients. Fold edges of foil to seal, then barbecue for 20 minutes.
Flank steak is an underrated lean cut of beef that's exceptionally flavourful and made for marinating. Skirt steak is similar, but marbled with more fat than a flank steak. Avoid overcooking to maintain tenderness. In France, both skirt and flank cuts are called Bavette. For best results with a flank steak, cut into thin slices at a slant against the grain before serving.