Prepare the red onion by trimming off the top and bottom ends, peeling away the outer skin, and then slicing it very thinly. You can use a mandoline for uniform slices, or a sharp knife.

Place the thinly sliced red onion into a heatproof jar or bowl. A 1-liter (quart-sized) jar works well.

In a small saucepan, combine the apple cider vinegar, water, granulated sugar, and salt. Stir to combine.

Heat the mixture over medium heat, stirring occasionally, until the sugar and salt are completely dissolved. Do not bring it to a boil; just ensure it's hot and well combined.

Carefully pour the hot liquid over the sliced red onions in the jar or bowl. Ensure the onions are fully submerged in the pickling liquid. You may need to gently press them down.

Let the onions cool at room temperature for at least 20-30 minutes. During this time, they will soften and turn a vibrant pink color. For a more intense pickle, you can let them sit longer.

Once cooled, transfer the jar to the refrigerator. The pickled onions are best after at least 30 minutes in the fridge, but can be stored for up to 2-3 weeks.