In a large bowl, combine the spring onions, lemon grass, coriander, mint leaves, lime juice, fish sauce, chilli sauce and sugar and stir until the sugar is dissolved. Adjust the flavour, if desired, by adding more sugar and/or fish sauce. Set aside.
Cook the steak over high heat on a preheated barbecue for approximately 4-6 minutes on each side, until it is cooked medium. Do not overcook. Remove from heat and slice into thin strips. Add the meat and its juices to the sauce and refrigerate, covered, for at least 3 hours.
Tear the lettuce into bite size pieces and place in a salad bowl. Arrange the cucumber on top and then add the meat and sauce. Top with the cherry tomatoes and garnish with fresh coriander leaves.