Cut the cucumber lengthwise into 1/4-inch pieces, cut each piece in half, then cut the halves lengthwise into thin strips. Place them into a bowl and set aside.
Place the soy sauce, mirin, rice vinegar, and sugar into a small container with a secure lid. Put the lid on and shake to create the vinaigrette dressing.
Pour the vinaigrette over the cucumber, toss to coat, cover, and marinate for 1 hour, stirring once or twice.
Just before serving, sprinkle the toasted sesame seeds on top.Servings: 4