Preheat the oven to 180°C (160°C fan-forced).
Wash the octopus tentacles under cold water, place them in a non-stick roasting tin, cover the tin with a layer of baking paper and then foil, and secure the edges around the roasting tin to seal it. Cook for 40-45 minutes, or until the octopus is tender when tested with a fork. Check it once or twice during the cooking time to ensure it doesn't dry out. If necessary, add a little water for moisture.
Place the tentacles in a large bowl, add the oil and squeeze over the lemon juice. Chop the lemon into quarters and add to the bowl. Add the garlic, rigiani and salt and season with some freshly ground black pepper. Cover and marinate for 30 minutes or overnight in the refrigerator.
Preheat a large barbecue flat plate or chargrill plate to hot. Drain the octopus and cook for a few minutes on each side. If the flat plate or chargrill is small, cook the octopus in two batches, reheating the plate to hot between batches. Meanwhile, to make the slaw, combine the cabbage and carrot and dress with the wine vinegar and oil to taste. Slice the octopus tentacles to serve.
Serve with lemon wedges and the cabbage slaw.