- STEP 1
Place the fish fillets in a pan, add the stock and cider, and bring to a very gentle simmer. Poach for 5 minutes, then remove the fish with a slotted spoon. Reserve the poaching liquid. Preheat the oven to 200C/gas mark 6. Flake the drained fish into 4 x 500ml individual pie dishes (or use a 2-litre square ovenproof dish) and mix with the seafood or prawns.
- STEP 2
Heat 1 tablespoon of the oil in a pan and fry the onion for 3 to 4 minutes over a low heat to soften. Pour in the poaching liquid. In a small bowl, mix the butter and flour together to form a smooth paste. Add this to the hot liquid, a little at a time, whisking constantly until the paste is completely incorporated and the sauce is thick and smooth. Stir in the parsley, then divide the sauce evenly between the pie dishes.
- STEP 3
For the topping, rinse the potato slices in cold water to remove excess starch. Drain and pat dry with kitchen paper. Place the potato slices over the fish, scattering the sliced leeks evenly among them as you go. Drizzle the remaining oil over the potatoes and bake for about 35 minutes, until they are golden and soft.