Heat oil in a fairly large pan (preferably a wok) and add the onion until it softens. About 2-3 minutes. Then add the tuna and sauté for another 3 minutes.
Add the chopped tomatoes, tomato puree, Italian herb seasoning, oregano, garlic granules and salt & pepper to taste.
Simmer for about 15 minutes-20 minutes or until the sauce has thickened stirring from time to time.
While the sauce is simmering, heat the oven at 200 degrees or 180 for fan-assisted ovens. And cooked the pasta shells 2 minutes less than the directed time on the packet. It needs to be a little bit tougher as it still needs to be cooked in the oven. DO NOT OVERCOOK. Drain and set aside.
When the sauce has thickened add the chopped fresh basil and the drained pasta shells and stir until thoroughly mix.
Transfer into an oven-proof dish and topped with grated cheddar and mozzarella. Bake for another 20- 25 minutes or until the cheese on top has turned golden brown.