Chop onion in half lengthwise. Trim ends and remove papery skin.

Slice each half into thin 1/2 moons. I like them as thin as I can get them with my knife, but no need to stress about it.

Place onion slices in a glass jar. Cover with red wine vinegar (you may need more or less depending on the size of your jar and onion.

Sprinkle with a generous pinch of fine salt.

Eat straight away but they're better if you can wait 10 minutes or longer.

Will keep in the fridge for a month or more.