Thinly slice the red onion. Use your hands to make sure all the pieces are separated and add them to a glass jar.
In another bowl or cup, add the vinegar, salt, sugar and peppercorns. Pour the boiling water over the top and stir so that the sugar and salt dissolve.
Pour this over the red onions and make sure they are fully saturated. If you need more, add equal parts vinegar and hot water until the onion pieces are fully submerged.
Seal the jar, give it a shake and let it sit on the counter for 1 hour. Then move it to the fridge. Within 4 hours the onions should be bright pink and ready to go!