Thinly slice the red onion using a knife or mandolin.
Add the vinegar, water, brown sugar and salt to a saucepan to make the brine.
Heat over medium heat and stir until the sugar and salt dissolve. Remove from heat.
Add the sliced red onions, black pepper, bay leaf and crushed red pepper flakes to a heat-proof bowl or jar.
Pour the brine over the sliced red onions until they are fully covered. If you are short on brine add a splash of water and vinegar until the onions are submerged.
Let the onions sit for at least 15 minutes to allow the flavors to infuse. The longer they have to marinate; the more intense the flavors will become.
Notes
Tip: turn the vent fan on over your stove when you are heating up the vinegar.
Tip: save the pickling bring after you finish eating the onions. You can use this brine for one more batch of pickled onions (simply add sliced red onions to the brine). Discard the brine after eating the second batch of onions.
Substitutions - use any vinegar you like such as white vinegar, red vinegar or rice wine vinegar. You can also try this recipe with different varies of onions such as yellow, white or sweet onions!