Step 1

Preheat the oven to 200°C. Place fish, milk, onion, bay leaf and 1 tbs tarragon in a deep frypan over medium heat. Season and bring to the boil, then reduce heat to low and cook for 5 minutes or until flavours are infused. Remove the fish and set aside. Strain the milk into a jug.

Step 2

Melt 50g butter in a saucepan over medium heat. Add flour and cook, stirring, for 2-3 minutes until pale golden. Whisk in milk until smooth and cook, stirring, for 2 minutes or until slightly thickened. Stir in remaining 2 tbs tarragon and season.

Step 3

Meanwhile, place potato in a pan of salted water and bring to the boil, then cook for 15 minutes or until tender. Drain, then return potato to the pan and cook for 30 seconds to remove excess water. Mash the potato with 75g cheese and the remaining 25g butter, then season.

Step 4

Flake the fish, discarding the skin, and place in a 2L (8-cup) ovenproof dish with the prawns and sauce, stirring to combine. Top with the mashed potato, spreading it carefully to cover the filling. Scatter over the remaining 25g cheese and bake for 30-35 minutes until bubbling and golden.

Step 5

Allow to cool for 5 minutes, then serve.