Preheat the oven to 170℃ or 350℉
Melt the butter in a medium sauce pan (if using the onion add it now, and let it sweat gently for about 5 minutes before adding the cornflour) If not using the onion, add the cornflour.
Mix to make a thick paste and let cook for about 2 minutes.
Slowly add the milk and scrape the bottom of the pan, loosening any brown bits.
Bring the milk to the boil and let simmer for about 2 minutes until the mixture is thickened.
Add the frozen peas and the fish mix. Let simmer gently for a further 5 minutes.
Add the chopped parsley.
Place the fish mixture into an oven proof baking dish.
Top with the celeriac mash and some fresh pepper.
Bake in the oven for about 20 minutes.