Peel your red onion and cut it in half from root to tip
Stuff the onion slices into a clean wide-mouth mason glass jar
If you like you can also add garlic cloves, jalapeno slices, bay leaves, or pepper flakes into the jar.
In a medium saucepan over medium high heat combine apple cider vinegar, sugar, water, and salt.
Bring the mixture to a boil as you stir to ensure the sugar and salt has dissolved into the liquid.
Remove the pickling liquid from the heat and let it stand for 10 minutes to cool down
Pour the warm brine over the onions, making sure every slice is submerged. Press the onions down if necessary
Cover and refrigerate for at least 8 hours to overnight.
Serve and enjoy