Chop the onions, pour in a bowl and add enough water to cover. Add a few drops of the lemon into the water. Leave for 10 mins, then pour in a colander to drain.
Wash and chop the tomatoes, coriander, cucumber, avocado and chilli.
Add into a bowl with the chopped onions. Pour in the lemon juice, extra virgin olive oil and a pinch of salt and toss.