Chop the tomatoes and mix them with the onion. Season well and add lime juice to taste.
Spoon some of the tomato mixture over each chapatti, top with the tuna and half the cheese. Roll up each chapatti and arrange them in a heatproof dish. Sprinkle over the remaining cheese and any remaining tomato salsa.
Cover and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes until golden. Garnish with coriander and serve immediately.