1. Take the fish and remove any scales, bones or skin that the fishmonger may have left on. Cut the fish into even 4cm cubes and distribute evenly in the bottom of 6 by 8 inch rectangle baking dish.

2. Wash and slice the leeks into 1cm rounds and gently sauté in olive oil over a medium heat for approximately 5 minutes until just cooked, season well and set to one side.

3. To make the sauce melt the butter in a pan and add the flour.

4. Stir until the mixture forms a smooth paste which leaves the sides and base of pan cleanly.

5. Cook for a minimum of 2 minutes to cook out the taste of the flour.

6. Pour in 100ml of chicken stock and allow to boil without stirring.

7. Stir until the mixture blends smoothly and beat vigorously, and repeat this process with the remainder of the stock and cream, beating well after each addition to avoid getting any lumps.

8. Once the sauce had reached a thick smooth consistency, remove from the heat and season well. Add the lemon zest and juice, chopped tarragon and cooked leeks and mix well.

9. Pour the sauce over the uncooked fish and leave to cool.

10. Peel and boil the potatoes in salted water for 10 to 15 minutes until tender, drain and then mash well.

11. season well and add the cream, grated parmesan cheese and diced butter and mix well until the mashed potatoes are smooth and creamy

12. Gently spoon the mashed potatoes over the sauce and use the edge of a palette knife to make a dented pattern in the mashed potato by dragging it along in lines and pushing down every few centimeters.

13. Brush with melted butter and bake at 170°c/325F for 40 minutes until the potato is golden brown and crispy.

14. Serve the fish pie with steamed green beans.

Chef tip
To make individual fish pies divide the fish, sauce and mashed potato between six individual baking dishes and cook for 30 minutes
Try experimenting with different types of meaty fish such as Pollock, sword fish and halibut