Chop red onion in half. Peel and slice very thin into half-circle slices. Use a mandolin if you have one.
Add the onions into a bowl with warm water, and lightly squeeze the onions to release the onion flavor. Drain the water.
Place the onions into a jar. I use a 35.9oz Weck Jar, or you can use a wide mouth 32 oz mason jar. You want to have enough space for everything to fit. In a measuring cup add the salt, hot water, and rice vinegar. Mix with a spoon. Pour on the onions.
Let cool on the counter. Then cover with a lid and refrigerate for at least 2-4 hours, or leave them overnight in the refrigerator for the best results. (Reuse the liquid up to 4 times. Adjust vinegar and salt each time if needed.)
Notes
TIP: Reuse the liquid up to 4 times. Make sure to adjust the salt, vinegar and water ratio to taste when you do.VINEGARS: Make sure your rice vinegar DOES NOT have any SUGAR. This will change the flavor. I would NOT recommend apple cider or white vinegar. They are just not the perfect fit for this pickling liquid. I've tried them with both and found the flavor to be overpowering, which will not give you the intended flavor meant for this recipe.