Take the salt, apple cider vinegar, sugar, and warm water and combine them in a medium bowl.
Whisk the ingredients together until the salt and sugar have dissolved fully.
Place the sliced onions in each jar. I had 8 oz mason jars on hand, so that is what I used. I was able to fill 5 mason jars.
After placing the sliced onions in a jar, add the mustard seeds, peppercorns, and bay leaves and pepper flakes (if using) so that the spicing ingredients. *EACH jar got 1 tsp of mustard seeds + 5 peppercorns - dash of red pepper flakes - I gave 3 jars bay leaves
Pour the above mixture over the sliced onion. Let the jar sit for 1 hour at room temperature for a quick pickle. If place the jar in the fridge overnight, they will have more flavor.
Enjoy on salads, sandwiches, burgers, tacos and just about everything else you can think of!
Notes
Keep the pickled onions refrigerated so that you can enjoy them for up to three weeks. If they last that long! Mine were gone in days!