The easiest way to make guacamole is in a large, heavy pestle and mortar or, in Mexico, a molcajete. Put the chillies and onion into the bottom of the pestle and crush to a paste with a couple of teaspoons of salt.
Stir in the avocado flesh, plums and half the grapes. Season to taste with the lime juice. Serve at room temperature decorated with the remaining grapes, with tortilla chips or pork scratchings.
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