Peel, chop and boil the potatoes until soft. Mash with the butter and set aside.
Warm the milk in a small saucepan along with the bay leaf and half the onion. Add the fish pie mix and simmer for 5 minutes. Remove the fish and set aside. Discard the onion and bay leaf, but retain the milk.
Slice the other half of the onion and the leek and fry gently in the olive oil in another pan until soften. Add the flour and heat for 1 minute, stirring all the time.
Add the milk to the onion/leek mix a spoonful at a time stirring all the time, until you end up with a thick sauce. Stir in the peas and the fish.
Pour the mixture into an ovenproof dish, top with the mashed potato and bake in the oven at 180 (fan) for 25 minutes.
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