Finely chop the onion then put in a strainer or sieve and pour some boiled or cold water over it to (this will remove the harsh bite). Drain well.
In a small mixing bowl whisk together olive oil, lime juice, garlic, salt and pepper.
Dice the tomatoes (remove the seeds if you like). Deseed the chilli, then chip finely. Peel the avocado and cut the flesh into tiny cubes.
Put the diced vegetables into a serving bowl, add the onion and chopped coriander leaves too. Pour the dressing over and toss gently. Taste and adjust the seasoning/heat.