You will need: red onions, black peppercorns, apple cider vinegar, kosher salt, water, sugar, and red pepper flakes.
Using a mandolin or food procesor, slice the onions really thin.
Super easy, just watch your fingers.
Nice and thin.
Place the sliced onions in a heat safe bowl or a jar that is easy to pour into. Put this aside.
In a small saucepan, bring water, apple cider vinegar, salt, peppercorns, red pepper flakes, and sugar to a boil.
Stir until the sugar and salt dissolve.
Pour the hot liquid over the sliced onions carefully.
Press the onions down if needed to make sure that they are completely submerged in the brine.
Once cooled, cover the onions and store them in the fridge for up to three weeks.
Enjoy with tacos, burgers, bagels - anything you can think of!