Preheat a fan forced oven to 200°C. Grease an 8-cup capacity ovenproof dish. Cook pasta in a pot of boiling, salted water, according to the packet instructions. Drain and put aside.
Melt butter in a saucepan over medium heat and whisk in the flour until combined. Remove from heat. Gradually add the milk, whisking constantly to ensure there are no lumps. You want it to coat the back of a spoon but not be too thick so adjust the milk accordingly. Season well.
In a saucepan, melt 1tsp of butter over a medium heat and add the onion. Stir constantly for a few minutes until softened. Take off the heat and combine in a bowl with the corn, tuna and white sauce.
In a large oven proof dish, combine the pasta and sauce until well combined. Top with the cheese and put in the oven for 25 minutes or until the cheese is melted and golden.