Slice the ends off a washed English cucumber, then slice it in half length-wise, and then slice each half into thin half moon shapes. Set them aside in a large bowl.
Dice the cilantro and scallions and set aside.
In a bowl add the 1/4 cup of rice vinegar, 1 tsp of toasted sesame oil, 1/2 teaspoon of kosher salt, 1/4 teaspoon of red pepper flakes, 1/4 teaspoon of sugar, and mix everything very well. If you have a jar or container with a lid, shaking the dressing works very well.
Pour the dressing over the cucumbers and mix well.
Add the cilantro and scallions to the cucumbers and mix again.
Lastly, sprinkle some toasted sesame seeds over the top of the salad.
Serve the salad immediately or let it sit, covered, in the fridge for an hour or two. It wil actually taste even better after it sits for a bit!