Pull apart the pomelo (see below) or segment the grapefruit, retaining any juices for the salad. (Pomelos may be dry, which is why the recipe gives lime juice as an option.)
Put 4 tbsp juice into a small pan with the fish sauce, sugar and tamarind paste. Bring to a simmer for 2 mins, or until thick and syrupy, then leave to cool.
Halve, stone, then slice the avocado, and toss with the shallot, chilli and remaining juice. Mound into bowls with the pomelo flesh, prawns, lettuce and coriander. Spoon over the sticky dressing, then scatter with the chopped peanuts.
