Preheat oven to 200C. Place the salmon, white fish, leek, carrot, milk and bay leaf in a large, deep frying pan over medium-low heat. Bring to a simmer and cook for 2 mins or until fish is almost tender. Remove from heat. Use a slotted spoon to transfer the fish and vegetables to a large heatproof bowl. Set aside to cool slightly. Strain the poaching liquid into a jug. Add solids to the fish and vegetables in the bowl, discarding bay leaf.
Heat butter in a saucepan over medium heat. Add flour and cook, stirring, for 2 mins or until mixture bubbles. Add milk mixture and cook, stirring constantly, for 5 mins or until sauce boils and thickens. Remove from heat. Set aside for 15 mins to cool.
Coarsely flake the fish. Add to the cooled sauce mixture with the reserved vegetables, cream, peas, chives and dill. Season. Spoon evenly among four 1 1/2 cup (375ml) ramekins. Place on a baking tray.
Use a sharp knife to cut 1 sheet of pastry into quarters. Use a sharp knife to lightly score pastry, being careful not to cut through. Place over filling, allowing corners to overhang. Cut fish shapes from the remaining pastry sheet. Place on top of scored pastry. Brush the pastry with egg. Bake for 12-15 mins or until the pastry is golden brown and puffed. Set aside for 5 mins to cool slightly. Serve with mash.
