Slice the red onion as thin as possible using a mandolin, then place the slices in a shallow bowl or wide-mouth mason jar. Set the onions aside.
1 large red onion
In a small saucepan -- bring water, apple cider vinegar, salt, peppercorns, red pepper flakes, and sugar to a boil.
½ cup apple cider vinegar, 2 teaspoons cane or granulated sugar, 1 ¼ teaspoon kosher salt, 12 whole peppercorns, ¼ teaspoon crushed red pepper flakes, 1 cup water
Stir until the sugar and salt dissolve. (Try not to stand over this/breathe it in as it boils - the vinegar will be pungent!)
Pour the hot liquid over the sliced onions carefully then press the onions down to make sure that they are completely submerged in the brine.
You can slide the handle of a spoon or fork into the jar to pop any air bubbles.
If the onions are not going to be eaten right away, let them sit at room temperature until they cool off. This should take 30 minutes - 1 hour. Once cooled, cover the onions and put them in the fridge.