If you have a mandoline, now's the time to use it; if not, use a vegetable peeler to make the long, thin ribbons. The salt will draw water out of the cucumber, so make this right before you're ready to eat. Serves 3-4.
1 large, very firm English (seedless) cucumber
1/4 cup Japanese rice vinegar
1 tsp sesame oil
1/2 tsp sugar
1/4 tsp salt
1 pinch red pepper flakes
black sesame seeds, for garnish
Trim the ends off the cucumber. Cut the cucumber in half to make two pieces approximately six inches long. Using a mandoline or a vegetable peeler, slice the cucumber lengthwise into thin ribbons, and place in a mixing bowl.
Combine the vinegar, sesame oil, sugar, salt and red pepper flakes in a small glass measuring cup, and stir well to dissolve the sugar. Taste, and adjust seasoning to your liking. Pour over the cucumber ribbons, and toss well, but very gently, to thoroughly coat the cucumber. Place in a serving bowl, and garnish with black sesame seeds.