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750g potatoes (approx 3 large)
2 leeks, finely sliced
1 garlic clove, crushed
3 tbsp plain flour
50ml dry white wine
400-450ml semi-skimmed milk
175g Cheddar cheese, grated
1 tsp mustard powder
500g fish pie mix (from the fish counter, or thawed from frozen)
150g raw king prawns
Preheat the oven to 200°C, gas mark 6. Place the whole unpeeled potatoes into a pan and cover with boiling water. Simmer for 15-18 minutes until tender but not soft when poked with a knife. Drain and submerge the potatoes in cold water for at least 10 minutes.
Place a large pan or casserole dish over a medium heat. Add half the butter and allow to melt, then tip in the leeks. Sauté until soft, then stir in the garlic and cook for a further 2 minutes. Add the remaining butter and allow to melt.
Sprinkle the flour onto the leeks and stir until coated, then add the white wine. Cook for 1-2 minutes, then gradually add the milk, stirring all the time, until a thick, smooth sauce forms – you may not need all the milk. Season well and stir in 100g Cheddar cheese and the mustard powder.
Once the cheese has melted, add the fish and prawns to the sauce. Stir to combine, then transfer to a large ovenproof serving dish and set to one side while you make the topping.
To make the rösti topping, peel the skin off the potatoes (it should come away easily) and grate coarsely into a large bowl. Stir through the olive oil and season well. Spread the rösti over the fish, filling in an even layer and making sure it covers the mixture right to the edges. Sprinkle over the remaining 75g cheese and bake for 25-30 minutes, or until piping hot throughout and the prawns are pink and the fish cooked through, with a golden and crunchy topping. Serve with a side of buttered greens.
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