Mix lime juice, red wine vinegar, sugar, onion and finally-chopped chilli (with seeds, if you like it hot) in a non-metallic bowl. Cut up mackerel in bite-size chunks, add to the mix and stir gently. Put in the fridge for no more than 15 minutes. Take out of the fridge, stir again, add mint and coriander to taste, along with chopped avocado. Season with salt, and serve immediately. This dish cannot be left to cure for too long, as the fish will over-marinate. If you like a little more heat, add a dash of your favourite hot sauce, such as tabasco.