Preheat the oven to 180°C (160°C fan forced). Lightly grease a medium sized (33x22x5 cm) baking dish with oil.
Bring a large saucepan of salted water to a boil over high heat. Add the shells and cook, stirring often to ensure they don’t stick together, for about 8 minutes, or just tender. Drain.
Meanwhile, heat a large saucepan over medium heat. Heat up the tin of mushroom soup for approximately 3-5 minutes.
Stir in the pasta, tuna, mushrooms, spinach, and 1/2 cup of the Parmesan cheese. Season to taste with salt and pepper. Transfer the mixture to the prepared baking dish.
In a small bowl, mix together the breadcrumbs and remaining 1/2 cup Parmesan cheese. Sprinkle over the pasta mixture.
Bake for about 25 minutes, or until bubbly all over and very pale golden on top. Stand at room temperature for 5 minutes before serving.