1. Whisk together the vinegar, sugar, coriander, red pepper flakes, and salt in a salad bowl until the sugar is dissolved.
Cut cucumbers lengthwise, scoop our seeds and slice, finely dice red onion, slice tomatoes, chop cilantro, and chop mint, and toss to coat with dressing. Cover and refrigerate for 1 hour to blend the flavors.
2. Before serving, toss again, plate on bed of romaine garnish with sprigs of fresh mint.