Preheat the oven to 375°F (190°C). Put the potatoes in a pot, cover with boiling water and bring back to a boil. Reduce the heat and simmer for 10 minutes until just tender. Drain.
Using kitchen scissors, cut the fish into large chunks and arrange in the base of a greased, large shallow ovenproof dish. Season with pepper and sprinkle the chives over the top. Pour in the hot stock, then scatter with the peas and leeks.
Thinly slice the parboiled potatoes and arrange in an even layer over the fish and vegetables. Season again with freshly ground black pepper. (There's no need to season with salt as the fish is quite salty.) Cover with a lid or a piece of greased foil and bake for 30 minutes.
Remove the dish from the oven and increase the oven temperature to 450°F (230°C). Sprinkle the cheese over the potatoes, then return the dish to the oven and bake for a further 5-10 minutes until crisp and golden. Serve.
Cook's tip: To prepare ahead, use chilled rather than hot stock to assemble the dish, then cover tightly and refrigerate for up to 12 hours before cooking. Add an extra 5 minutes to the cooking time.
Each serving: 31.5 g protein, 6 g fat of which 3 g saturates, 25 g carbohydrate, 8 g fibre, 296 Calories