1) Preheat oven to 220°C/ gas mark 7.
2) Place bread in food processor and pulse for 30 seconds into bread crumbs.
3) Heat oil in a large frying pan over medium heat. Add onions and cook, stirring, until onions are soft and translucent, eight minutes. Add celery and cook, stirring, until just tender, six minutes. Add mushrooms and cook, stirring, until mushrooms release their water, five to seven minutes.
4) Add flour all at once and stir immediately and vigorously with a wooden spoon until flour is completely incorporated into vegetable mixture until there are no flour lumps. Add milk and broth, stir to combine and bring mixture to a boil, stirring frequently. Reduce heat to a vigorous simmer and cook, stirring, until liquid has thickened and reduced by about 120ml, seven to eight minutes. Add salt and pepper and stir to combine.
5) Combine cooked pasta, vegetable-liquid mixture, broccoli, peas and tuna and toss to incorporate. Pour into a 20 by 30cm casserole dish. Top with bread crumbs.
6) Bake for 25 minutes, or until crumbs are golden brown and toasted.
Calories 500; Total Fat 7 g; (Sat Fat 1.5 g, Mono Fat 2 g, Poly Fat 1 g) ; Protein 49 g; Carb 56 g; Fiber 11 g; Cholesterol 75 mg; Sodium 990 mg