Peel the onion and deseed 1 chilli, then roughly chop it all on a large board. Destone the avocados and scoop the flesh onto the board. Start chopping it all together until fine and well combined. Pick over most of the coriander leaves, roughly chop and add the tomatoes, then continue chopping it all together. Add the juice from 1 lime and 1 tablespoon of oil, then season to taste with sea salt, black pepper and more lime juice, if needed. Deseed, finely chop and scatter over the remaining chilli if you like more of a kick. Pick over the reserved coriander leaves, then serve.