In a small bowl, stir together garlic, sugar, red-pepper flakes, vinegar, and water.
In another bowl, toss together carrots, scallions,
and cilantro leaves. Toss with half the vinegar mixture.
Split and lightly toast hero rolls; dividing evenly, layer with carrot mixture and sliced flank steak. Drizzle with remaining vinegar mixture, if desired.
A traditional Vietnamese sandwich is made with pork, but in this beef variation, the flavors are the same.
If you don't want to make steak from scratch, you can use an equal amount of sliced roast beef in its place.