Preheat oven to 200°C/180°C fan-forced. Grease an 8-cup capacity ovenproof dish. Cook pasta in a saucepan of boiling, salted water, following packet directions. Drain.
Melt 30g butter in a saucepan over medium heat. Add flour. Cook, stirring, for 1 minute or until bubbling. Remove from heat. Gradually stir in combined cream and milk. Return to heat. Cook, stirring constantly, for 3 minutes or until sauce boils and thickens. Remove from heat. Stir in tasty cheese. Season.
Meanwhile, melt 20g butter in a small frying pan over medium heat. Add onion. Cook, stirring, for 5 minutes or until softened. Transfer to a bowl. Add tuna, pasta and cheese sauce. Mix well. Spoon mixture into prepared dish. Combine breadcrumbs and parmesan in a bowl. Sprinkle over tuna mixture. Dot with remaining butter. Bake for 30 minutes or until cheese is melted and golden. Serve.
