Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a frying pan, add the onion and fry, stirring, for 5 minutes or until just beginning to turn golden around the edges.
Stir in the garlic, red pepper, paprika and dried chillies and cook for 2 minutes. Mix in the tomatoes, stock and a little salt and pepper. Bring to the boil, then tip into the slow cooker pot. Break the tuna into large pieces and stir into the tomato mixture. Cover with the lid and cook on low for 4–5 hours.
When almost ready to serve, bring a large saucepan of water to the boil, add the spaghetti and cook for about 8 minutes or until tender. Drain and stir into the tomato sauce. Spoon into shallow bowls and sprinkle with grated Parmesan and basil leaves to taste.