- Put the sesame seeds in a dry small fry pan, set over medium-low heat and toast the seeds, shaking the pan frequently, until golden brown, about 5 minutes. Transfer to a plate and let cool.
- Alternatively, put the seeds in a mesh spice toaster and hold it about 5 inches above a medium gas flame on the stovetop; do not place your hand too close to the flame. Shake the seeds in the toaster until golden brown, 30 to 45 seconds. Transfer to a plate and let cool.
- Using a mandoline or a sharp knife, cut the cucumber into 1/8-inch slices. Transfer to a colander, sprinkle with the 1/2 teaspoon salt and let stand for 10 minutes.
- In a small bowl, whisk together the vinegar, sugar and sesame oil. In another bowl, combine the cucumber, scallions, shiso and sesame seeds. Add the vinaigrette and stir to combine. Season with salt and pepper. Serve the salad at room temperature or chilled.