Put the pasta in a large pan of boiling water and cook following pack instructions. Meanwhile, put the oil from the can of tuna in a saucepan with the onion and fry for 5 mins. Pour the milk on top of the onions and whisk in the flour. Keep cooking, whisking all the time, until the sauce thickens. Off the heat, stir in the tuna, 100g of the cheese and the mustard powder.
When the pasta is cooked, drain it and tip into the sauce along with the sweetcorn. Season, then pile into a large ovenproof dish.
Heat the grill to high. Tuck the tomato slices in and around the pasta so they stick out, then scatter the remaining cheese over the top. Grill for 5-10 mins until golden, crispy and bubbling.