Step 1: Slice the red onion into thin rounds or half-moons using a sharp knife or mandoline.
Step 2: In a small saucepan, combine the water, apple cider vinegar, white vinegar, sugar, and salt over medium heat.
Step 3: Whisk the mixture until the sugar and salt are completely dissolved, then remove from heat just before it reaches a boil.
Step 4: Pack the sliced onions tightly into a clean glass jar.
Step 5: Pour the warm brine over the onions until they are completely submerged.
Step 6: Let the jar sit at room temperature for 30 minutes, then cover and refrigerate for at least 1 hour before serving.