Chop onions into thin long slices
Pack onions slices into a heat proof pint sized jar
Bring water, vinegar, sugar, and salt to a boil in a saucepan
Mix sumac into the hot vinegar liquid and pour the liquid the over the onions
Cool the onions to room temperature, and then store in the fridge for 12-24 hours to let the flavors meld. Onions will keep in the fridge for several months.