1 med-large onion (red or white) thinly sliced
1/2 cup apple cider vinegar
1 TB cane sugar
1 1/2 tsp salt
1 cup warm water
Stuff the onion slices in to a jar (mason jars work great).
In a bowl, dissolve the sugar and salt with the water and apple cider vinegar. Pour mixture over onion slices, seal with an airtight lid, and let sit for at least an hour.
Store in your fridge for up to three weeks.