Roast steak on each side until medium or cooked all the way through. Remove and set aside for 5 minutes. Using a sharp kitchen knife, cut into 5 mm thick slices.
Heat water in a wok, add meat and do over medium heat for 2 min, no longer. Transfer meat and liquid to a suitable bowl (no metal vessel!).
Add fish sauce, juice of one lemon, lemongrass, onions, cilantro and mint and mix well. Cover and marinate in the refrigerator for 2 hours.
Fold in the chopped cucumbers. Bring the leafy salad to the table on a bed of cabbage and garnish with additional mint, if desired.
Preparation Tip:
If you would rather have your steak well done, simply roast longer.