1. Mix the onion, garlic, coriander and chillies, and chop very finely (or whiz together in brief bursts in a proccessor, taking care not to end up with a purée).
2. Peel and stone the avocados and mash with a potato masher or fork (do not put them in the processor, as you want to end up with slightly uneven mush).
3. Mix the avocados with the chopped flavourings, tomato, juice of 1/2 lime, a pinch of sugar, salt and pepper. Taste and add more lime, if needed. There should be just enough to take the edge off the richness of the avocado, without making it too tart.
4. Scoop into a bowl, smooth and cover the surface with cling film to keep the air out and so delay browning.
5. Before serving, give it a light stir to reveal the vivid colour of the purée, then scatter with coriander and a dusting of cayenne.