Slice the red onions as thin as you can, mandoline works great.

Stuff the sliced red onions in jar with lid of your choice.

In a measuring cup, combine apple cider vinegar, salt, sugar, and water. Stir to dissolve the sugar and salt. Pour this mixture over your sliced onions, making sure they are completely covered. Let them set for an hour. After an hour, cover and store in the fridge for up to three weeks.