Take a large bowl and place lemon grass, spring onions, mint leaves, coriander, chilli sauce, fish sauce, lime juice and sugar in it. Then, stir all these ingredients until the sugar gets dissolved. After that adjust the flavour to taste if desired, either by adding fish sauce or sugar. Then leave it aside.
Preheat barbeque about 4 to 6 minutes and then sauté the sirloin steak fillet over it at high heat from both sides until it gets medium cooked. But be careful it should not be overcooked. After that remove the steak from the heat and slice it into strips. Add this meat along with its juices to the sauce in the mixture created in Step 1 and cover it. Put it in the refrigerator for minimum 3 hours.
In the meantime, take lettuce and tear it into bite size pieces and put it in a salad bowl. Set the cucumber on its top and then add the sauce and meat in it. After filling it with the mixture of meat and sauce, top off with cherry tomatoes and for garnishing, use fresh leaves of coriander.