Prepare the squid, reserving the ink sacs. Cut bodies of squid into 4 cm squares and score with the point of the knife in a diamond pattern. Combine marinade ingredients in a pottery, glass or enamel-lined bowl and squeeze in the ink from the reserved sacs. Add squid, cover and stand for 4 hours at room temperature or overnight in the refrigerator. Make the sauce. Cook garlic in oil until pale gold, then add onion and cook, stirring, until onion is translucent. Add tomatoes and stir over medium heat for 3 minutes. Season with salt and pepper. Add squid and marinade to the sauce, bring the mixture to the boil, reduce heat and simmer, covered, for 10-15 minutes or until squid is tender. Serve with rice and chili-flavored vinegar. Serves 6-8.
TO PREPARE SQUID: Pull out the head of the squid with the tentacles from the mantle (body). Remove eyes, ink sacs and parrot-like beak at the base of the tentacles. Set ink sacs aside and discard eyes and beak. Pull the fins off the squid. Remove and discard any yellowish deposit and the long internal quill bone (which looks like hard cellophane) from the squid. Rub off the skin from the mantle and fins. Wash and dry the squid inside and out.
CHILI-FLAVORED VINEGAR: put 1 cup palm vinegar, Japanese rice vinegar or white vinegar into a glass jar and add 2 fresh or dried chilis, slit down the sides. Stand for at least 4 hours or up to 48 hours before removing chilis. If hotter vinegar is- desired, crush chilis slightly.