This recipe is an adaptation* of the recipe True Thai, by Victor Sodsook (see review)
 

Thai Cucumber Relish

Combine the following in a saucepan:

• 1 cup distilled white vinegar
• 1 cup white sugar
• 1 teaspoon salt

Bring to a boil, stirring until dissolved, boil gently for 1 minute, and remove from heat.  Let solution cool to room temperature.

Slice one large peeled cucumber and an onion in thin slices.  Pour the cooled vinegar "dressing" over the veggies and serve.
 

*The original recipe did not use an onion.  They also added 4-6 thai chilies, 1/2 cup peanuts, and 1/3 cup cilantro to the sliced cucumber before adding the dressing.