Cut the end of the onion flat; leave the root in tact; remove the onion skin
1 large red onion, thinly sliced
Place the onion flat-side-down on mandoline or cutting board and slice to 1/18th" thickness; transfer to large glass mixing bowl or glass jar
1 cup apple cider vinegar, 1 cup granulated sugar, 1 teaspoon kosher salt
In a medium saucepan on medium heat, combine vinegar, sugar and salt; stir and heat 3-5 minutes until sugar is completely dissolved; pour warm vinegar mixture over sliced red onions; cover and refrigerate for at least 2 hours
When ready to serve, use a slotted spoon or fork for best results
Notes
If you use a large red onion, this recipe yields about 1½ -2 cups of thinly sliced onions.