Heat the oil in a frying pan, add the onion and cook for 5 minutes over a medium heat, then add the mushrooms and cook for5 minutes, until softened. Stir in the sweetcorn and tomatoes and simmer for a few minutes.
Meanwhile, stir the pasta into a pan of boiling salted water and cook according to pack instructions, until just tender. Drain, then put it back in the pan. Set the grill to hot.
Stir the pasta into the sauce, then break the tuna into large flakes and gently mix in. Spoon into the serving dish. and sprinkle with the cheese.
If you want to brown the cheese you can put the dish (make sure it’s oven-proof) on a baking tray and grill for about 5minutes until the cheese has melted and turned golden brown.