In a large pan saute the onion. Once onions are tender add in the fish pie mix with the milk and simmer for 10 minutes. Add in the flour to thicken the sauce and add the chopped parsley to season. Set aside.
In a separate pan add the potatoes to boiling water until tender, sieve and mash.
In an oven proof dish, add the fish mixture and the wilted spinach, topping with the mash potato. Cook at 180C for 20-25 minutes.
Serve with peas.