Combine the vinegar, water, sugar, and salt in a small saucepan and bring to a gentle boil, stirring occasionally to dissolve the sugar and salt. When the syrup is clear and slightly thickened, after about 2 minutes, remove from heat and let cool to room temperature.

Peel the cucumber (if the skin is tender and unwaxed you can opt to leave some or all of the skin on), and quarter it lengthwise to make 4 long strips. Slice the strips crosswise into little triangles about 1/4-inch thick. Put the cucumber bits, the onion, the chopped cilantro and the cooled vinegar-syrup into a bowl and toss well.

Serve in individual portions and sprinkle with garnish.