Preheat the oven to 180C/350F/Gas 4.
Put the tomatoes, sugar, onion, 2 tbsp of the olive oil, the orange and lime zest and juice and the coriander into a bowl and mix thoroughly to combine.
Heat the remaining oil in an ovenproof frying pan over a medium heat and gently fry one of the tortillas.
While it's still in the pan, add a third of the grated cheese to the fried tortilla, then put a third of the tomato mixture on top. Put another tortilla on top and repeat the process with a third of the cheese and a third of the tomato mixture. Finish with the third tortilla.
Transfer the pan and its contents to the oven for 2-3 minutes or until the cheese has melted.
To serve, remove the tortilla stack from the pan, cut into wedges and put onto a plate. Spoon the remaining third of the tomato salsa around the outside of the stack and sprinkle the remaining cheese over the top.