Get everything, except the avocados, prepped when you get home. Just before you want to eat, peel and stone the avocados and put the flesh in a bowl. Don't worry about how pulpy you make the avocado, as it will be mashed. Dissolve the salt in the lime juice and pour it over. Then add the other ingredients and mash with a fork until you have a rough lumpy mixture. At all costs avoid turning it into a smooth puree; by its nature the avocado will be smooth anyway, but you want as many soft but form-holding clumps as possible.
Recipe courtesy of Nigella Lawson