Rosti-topped fish pie

• waxy potato↗300g g

  halved

• skinless coley↗250g g

  fillets (look out for frozen coley)

• milk↗300ml ml
• butter↗50g g
• leek↗1

  finely sliced

• flour↗25g g
• chopped parsley2 tbsp tbsp
• Dijon mustard↗2 tsp tsp

Step-by-step instructions

1. 1

   Cook the potatoes in boiling salted water for 5-7 mins, until almost tender but firm enough to grate. Drain and refresh under cold running water. Put the coley in a shallow saucepan and pour over the milk. Bring to a simmer and cook for 5 mins. Drain the fish, reserving the milk, then set aside.

2. 2

   Heat half the butter in a medium saucepan, add the leek, then cook for 5-6 mins until softened. Stir in the flour for 1 min, then remove from the heat. Gradually add the milk, stirring well between each addition. Return to the heat and stir until the sauce comes to the boil. Simmer for 2 mins, then stir in the parsley and mustard.

3. 3

   Heat the grill to high. Flake the fish into large chunks, fold into the sauce, then place in a small ovenproof dish. Coarsely grate the potatoes. Melt the remaining butter, toss with the potatoes, season and scatter over the dish. Place under a medium grill for 5-10 mins until the potatoes are golden and tender.