Place potatoes in a microwave safe bowl with ¼ cup water. Cover and microwave on high for 6 minutes or until tender. Drain, cool slightly.

Preheat oven to 200°C fan forced. Place cream and milk in a jug and stir to combine. Set aside.

Melt half the butter in a large saucepan over a medium heat until bubbling. Add onion and cook, stirring until softened. Add flour and cook, stirring for 1 minute.

Turn to low, gradually add milk and cream mixture, stirring between additions until smooth. Once all of the mixture has been incorporated, increase heat to medium. Stir frequently until sauce boils and thickens, about 5 minutes. Stir in mustard zest, cheese, dill and parsley until smooth. Season.

Add salmon and white fish to pan and gently stir. Simmer for 3 minutes. Grease a 10-cup capacity ceramic dish on an oven tray to catch any spills. Spread potatoes over the base of dish.

Spoon fish mixture over potatoes then spread egg quarters over the top. Sprinkle with capers. Set aside.

For the topping, melt remaining butter in a jug, brush each pastry sheet, one at a time, with butter then scrunch gently and arrange on top of pie.

Bake for about 25 minutes or until golden and fish is cooked. Serve with mixed greens and lemon wedges.