Using a mandolin slicer, cut the onion into thin, even slices. You can use a knife if you prefer, just try to get as even slices as you can.
Combine the vinegar and water in a microwave safe dish and heat on high for 2-3 minutes, until the water is hot but not boiling.
Stir the salt and sugar into the vinegar mixture until dissolved.
Place the onions into a 16 ounce canning jar. It’s okay to push them down, so they all fit. Add in the peppercorns. Pour the vinegar mixture over the onions.
Store the onion jar on the countertop and loosely cover with the lid until the jar has cooled to room temperature. The color will start to deepen. Cover the jar completely and store in the refrigerator.
You can add the pickled onions to salad, sandwiches, bowls and more. They are really versatile and add a nice pickled crunch to any dish.