Pre-heat the oven to 200ºC.
Cut the potatoes into 1cm wide slices, then cut the slices into 1cm wide chips and place in a bowl of cold water to rinse off the starch.
Tip the chips into a colander and drain away the starchy water. Place the chips on absorbent kitchen paper and pat dry.
Place the dry chips on a non stick baking tray and spray well with low calorie cooking spray. Place in the preheated oven for 20 minutes, turning the chips once. The chips will be slightly tender at this point but not completely cooked. Remove from the oven and set aside.
Place the fish in a medium frying pan with the milk, spring onion and lemon zest. Season well with salt and pepper and place over a low heat. Cover with a lid and simmer for 10 minutes, until the fish is just starting to flake. Don't stir otherwise the fish will break up.
Spray the ovenproof dish with a little low calorie spray and use to grease the dish.
Place the mushy peas in the ovenproof dish and spread out evenly.
Remove the frying pan from the heat, and using a large slotted spoon, remove the fish from the cooking liquid and place evenly on top of the mushy peas. Keep the cooking liquid in the frying pan as this will be used to make the sauce.
Mix the cornflour with the water until smooth. Tip into the cooking liquid in the frying pan and stir until evenly mixed in. Place over a low heat and simmer for 3 - 4 minutes, stirring, until slightly thickened.
Pour the sauce over the fish and mushy peas in the ovenproof dish.
Place the chips in a medium bowl and spray with low calorie cooking spray. Add the vinegar and season well with salt and pepper. Toss the chips to coat them well.
Place the chips evenly on top of the pie. Place on a baking tray and bake in the oven for 15 - 20 minutes. The pie should be piping hot and the chips should be tender throughout and lightly browned.
Serve at once. You can sprinkle the chips with a little more salt, pepper and vinegar if you wish.