In a small bowl whisk together hot water, vinegar, sugar and salt until dissolved.
Cut the red onion in half lengthwise. Lay the flat side of one half on the cutting board, slice thin slices crosswise to create half moon shape slices. Repeat with other half and place slices in a pint size (2 cup) glass jar.
Pour the vinegar mixture over the onion, making sure all onion slices are submerged.
Allow to rest on the counter for about an hour. Cover if you haven't already and store in refrigerator for up to 2 weeks.
