Preheat a barbecue or grill to medium (if you're using a grill, line the tray with foil). Put the beef on the grill rack and cook for 5–7 minutes on each side, turning occasionally. Leave the meat to rest for at least 5 minutes, then slice it crossways into strips.
Mix the beef, lemon grass, shallots, kaffir lime leaves, lemon juice, fish sauce, ground rice, chillies or chilli powder and mint leaves together in a bowl.
Line a serving plate with a few mixed salad leaves and spoon over the sliced steak. Serve immediately.