Begin to make the Danish cucumber salad by peeling the cucumbers. Add equal parts water and rice wine vinegar to a bowl. Add a tablespoon of sugar to the liquids and stir until the sugar has completely dissolved. Using a mandolin or a knife, thinly slice the cucumber into the liquids. Mix well. Add the shaved onions and mix well. Lay plastic wrap over the cucumber salad and press it into the mix so that no oxygen is able to get into the salad. Allow the salad to marinate over night and serve.