Preheat oven to 200°C/180°C fan-forced. Grease a baking dish.
Cook pasta in a saucepan of boiling water, following packet directions, until just tender. Drain. Transfer to a bowl.
Melt margarine in a saucepan over medium heat. Add flour. Cook, stirring with a wooden spoon, for 1 to 2 minutes or until mixture bubbles. Gradually stir in milk. Bring to the boil. Reduce heat to medium. Cook, stirring for 5 minutes or until thickened, add 1 cup cheese . Cook for 2 minutes or until combined and cheese melted.
Stir in tuna, onion, frozen vegetables, brocolli and pasta and stir until heated through. Spoon into prepared dish. Sprinkle with breadcrumbs and remaining cheese. Bake for 20 to 25 minutes or until golden. Stand for 5 minutes. Serve.