Make sauce by placing all ingredients in the bowl of a stick blender, or the small bowl of a food processor. Blitz until finely chopped. Adjust seasoning to taste
Toss cucumber, onion, salt and sugar together. Leave to drain in a sieve for 10 minutes, then pat dry. Toss with lime juice and chill until ready to serve.
Cut squid tube lengthwise through 1 layer to open out, then pat dry and score with a sharp knife in a crisscross pattern.
Cut again lengthwise to divide into 2 equal pieces. Skewer each piece with 2 skewers crossing diagonally from corner to corner. Rub squid with fish sauce and garlic. Set aside 5-10 minutes before cooking.
Heat barbecue grill to very hot. Cook squid for 1 minute each side — squid should char a little.
Serve squid on steamed jasmine rice with cucumber salad, and chilli lime sauce on the side for spooning over the squid.