1-Begin by slicing the red onions thinly: Begin by slicing the red onions thinly. A mandoline slicer can help achieve uniform thickness, which ensures even pickling. Place the slices in a bowl and set them aside.
2-In a small saucepan, combine the rice: In a small saucepan, combine the rice vinegar, sugar, salt, and water. Heat the mixture over medium heat, stirring until the sugar and salt dissolve completely. This step is crucial as it creates the flavorful brine that will infuse your onions.
3-Once the brine is ready, pour it: Once the brine is ready, pour it over the sliced onions in the bowl. Make sure the onions are fully submerged. Allow the mixture to cool to room temperature, which usually takes about 10 minutes.
4-Transfer the onions and brine into a: Transfer the onions and brine into a clean jar, seal it tightly, and refrigerate. For best results, let them pickle for at least one hour before serving. The flavors will deepen the longer they sit, so feel free to make them a day ahead!