Cook noodles until just tender. Drain and rinse in cold water and drain thoroughly again. Put noodles into a bowl and add vinegar, sugar, soy sauce, and salt. Mix well. Cover and chill until cold.

Before serving, peel and seed the cucumber. Cut into quarters lengthwise. Put noodles in bowls, top with some cucumber slices, sprinkle extra sesame seeds over the top and garnish with some sliced scallion greens.