Method

Sprinkle salt on the chopped onion in a bowl - this sweetens it and takes out some of the bitter juice

leave it for half an hour or so then pour the onion juice away, add all the other ingredients and give it a good stir

Stir in mashed avocado. I leave the stone in the top - it's supposed to stop it going brown but probably doesn't. Looks good though. between the lime and tomato juice it's probably ok but put cling on it to make sure