Heat oil in a large frying pan. Add leek and garlic, cooking for 2 minutes until tender.
Pour in milk and stock powder, simmer gently.
Combine cornflour with 2 tablespoons of cold water to form a smooth paste. Add to milk mixture and continue stirring over low heat until thickened.
Add fish, broccoli and peas to sauce and heat through. Transfer to a 2 litre ovenproof dish.
Preheat oven to 180°C.Lightly brush pastry sheets with extra milk and layer on top of each other. Cut widthways into 6 strips.
Scrunch each length of pastry and place on top of fish mixture. Spray lightly with oil and bake at 180°C for 10–15 minutes, until pastry is golden and fish and vegetables are tender.