Preheat the slow cooker if necessary; see the manufacturer's instructions. Rinse the squid inside and out, pulling off the tentacles and reserving. Drain and slice the bodies. Put the tentacles in a bowl, cover with clingfilm and chill in the refrigerator.
Put the onion, chorizo, mushrooms and red pepper into the slow cooker pot. Add the garlic, rosemary, tomato purée and sugar, then stir in the sliced squid.
Pour the tomatoes and wine into a saucepan and bring to the boil or heat in the microwave. Add a little salt and pepper, then pour into the slow cooker pot and stir well. Cover with the lid and cook on low for 3–5 hours or until the squid is tender.
When almost ready to serve, put the cornflour in a small bowl with a little cold water and mix to a smooth paste. Stir into the slow cooker pot, add the squid tentacles, then replace the lid and cook, still on low, for 30 minutes. Spoon into bowls and sprinkle with chopped parsley. Serve with thickly sliced bread or rice.