Place onion, chilli, salt, coriander and lime juice in a small processor and pulse to a coarse mix.
Peel, de-stone and mash avocados.
Add processed mix to avocado mash and check seasoning.
At this stage the finely diced flesh of a large tomato can be added if wished.
Recipe Notes
To avoid the mix turning grey on its exposed surface place in a bowl, gently tamp down and cover with adherent cling film eliminating as much air as possible.