Here's my recipe, adapted from numerous others found on YouTube:
1: Cut one red onion along the grain to minimize cell damage,
2: Simmer the onion slices in water a few minutes to soften them,
3: Put the slices into a 16oz mason jar,
4: Fill the jar half with white vinegar and half with water,
5: Add a pinch or two of Diamond Crystal salt and a pinch or two of cane sugar. 6: Refrigerate overnight.
The onions turn a beautiful shade of pink as they age and they can last for weeks without going bad.