Preheat oven to 190 C / Gas 5. Butter a medium baking dish with 1 tablespoon butter.
Fill big pot 3/4 full of cold water. Cover and bring the pot of cold water to a fast boil. Covering the pot of cold water with a lid will help bring the water to a boil faster. Add 2 tablespoons of salt. Add the pasta, all at once, to the boiling salted water, and keep the heat high to bring the water back to the boil as quickly as possible. After you add the pasta to the boiling water, stir with a long wooden spoon. Cook the pasta for 8 to 12 minutes, uncovered, at a fast boil. Drain immediately into a large colander, and then pick up the colander with its contents and shake it well to remove excess water. Refresh in cold water to stop it from over cooking.
Heat saucepan over medium heat. Melt the butter in a saucepan and don’t allow the butter to brown. Sauté the onion, celery and garlic, and cook 5 minutes, until tender. Mix corn flour into water and stir in the pan along tomato soup. Bring to the boil; add the mushrooms then cook gently stirring for five minutes. Mix in the flaked tuna and heat through. Season to taste.
For the sauce, melt the butter in a saucepan and sauté the onion until transparent. Add the flour and continue stirring the mixture all the time until smooth. Gradually pour in the milk and keep stirring until the sauce is thickened and smooth. Add the chopped spinach and stir until wilted. Remove from the heat and stir in the grated cheese.
In the grease baking dish, add the tuna and tomato mixed with the pasta, sweet corn, parsely and then the spinach sauce.
In a saucepan melt 2 tablespoons butter in a small bowl, mix with breadcrumbs, and sprinkle over the dish. Top with cheese.
Bake for 20-25 minutes in the preheated oven until the cheese on top is golden and starting to brown.
Set it aside to cool for 5 minutes and then serve.