To make topping, toss bread, pepitas, parmesan and oil in a bowl. Set aside.
Cook pasta in a medium saucepan of boiling water as directed on packet, until al dente (just tender). Drain.
Heat oil in a large saucepan over a medium-high heat. Add celery and onion. Cook, stirring for 3-4 minutes, until softened slightly. Add flour. Cook, stirring a further 1 minute.
Reduce heat to low. Gradually stir in milk. Add onion powder and mustard. Season with pepper. Stir constantly over a medium heat until mixture boils and thickens. Remove from heat.
Gently stir in peas and corn, tuna, pasta and parmesan. Spoon into lightly greased ovenproof dish (6 cup capacity). Sprinkle with topping. Bake in 180C oven (fan-forced) for 30 minutes or until topping is crisp and browned.

Don’t throw out stale wholegrain bread. It’s perfect for making crunchy toppings or can be processed into breadcrumbs and frozen for later use.
Use wholegrain breadcrumbs for binding meatballs, rissoles and vegetable patties.
Recipe can be prepared completely 1 day ahead. Keep, covered in the fridge. To cook, remove covering and bake as directed in recipe, allowing an extra 5-10 minutes cooking time to ensure it heats through to centre.