Chop the onion and garlic really small (they will be eaten raw). Blanch (scald) the tomatoes, peel, seed and dice the flesh.
Cut open the avocado, remove the pit, scoop out the flesh and mash it in a baking bowl with a fork. Drizzle with the juice of one lemon.
Mix the onion, garlic and tomato with the avocado. Season heartily with salt, black pepper, the cayenne pepper as well as the sugar. Top with parsley leaves.