Cut the pomelo in half and scoop out the segments and juice. Discard the pith and thick skin surrounding each segment and break the flesh into small pieces. Stir the toasted peanuts into the pomelo flesh and set aside to allow the flavours to blend.
Cook the prawns in a saucepan of lightly salted boiling water for 1–2 minutes, or until the prawns turn pink and are cooked through. Remove with a slotted spoon and drain well.
Add the prawns to the pomelo flesh with the grapefruit juice, fish sauce, shredded spring onions and mint.
Sprinkle the finely sliced chilli, crushed chilli or pepper and nutmeg over the salad and toss together. Line the inside of a bowl with the lettuce leaves and spoon in the salad. Chill until required.