1In a large bowl, stir together the green onions, red onions, lemon grass, cilantro, mint leaves, lime juice, fish sauce, chili sauce and sugar until well combined and the sugar is dissolved. Adjust the flavor, if desired, by adding more sugar and/or fish sauce. Set aside.
2Cook the steak over high heat on a preheated grill for approximately 4-6 minutes on each side, until it is cooked medium. Do not overcook the meat! Remove from heat and slice into thin strips.
3Add the meat and its juices to the sauce and refrigerate, tightly covered, for at least 30min.
4Tear the lettuce into bite size pieces and place in a salad bowl. Arrange the cucumber on top of the lettuce, and then pour the meat and sauce over.
5Top with the cherry tomatoes and garnish with fresh cilantro leaves. (Optional) Sprinkle chopped red hot peppers and lime zest on top.