Using a mandoline slice onions thin and soak in two changes of water for 30 min - 1 hr
Slice cucumbers thin, or to desired thickness. Place in cucumbers bowl with paper towel to blot out excess moisture. Remove paper towel.
In a mason jar, combine all ingredients together and shake the jar well to dissolve sugar and combine ingredients. OR in large bowl, combine all ingredients and whisk together until all ingredients are combined.
Toss dressing with cucumbers then top with red onions.