Boil your pasta of choice in plenty of salted boiling water till al dente, according to the timings stated on the packet.
Meanwhile, empty the tuna into the bottom of a casserole dish. Pour the tinned tomatoes into the dish along with the passata and purée and stir. Add a pinch of salt and pepper for seasoning. Drain the pasta and add to the dish and toss well.
To make the white sauce for the cheese topping, melt the butter in a saucepan. Once melted add the milk and bring to the boil. Add the flour and stir until all of the lumps are gone and it's smooth.
Pour the white sauce over the tuna and tomato mix. Grate the cheese over the top.
Bake in a preheated 180 C / Gas 4 oven for about 40 minutes or until golden.
Always preheat the oven as this allows for a better bake. Also the amount of cheese is just a guide. More or less can be used depending on your taste.