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Assemble the pie a day ahead, making sure the fish and sauce are cold before combining with the prawns and tomatoes, and the mash has cooled before topping the pie; chill.
Preheat the oven to 180°C, fan 160°C, gas 4. Place the haddock and white fish fillets in a shallow roasting tray (or dish) with the bay leaves. Mix together the stock and milk; pour over the fish, cover with kitchen foil and bake for 20 minutes.
For the colcannon, simmer the potatoes in boiling salted water for 15 minutes or until tender; drain and return to the pan. Meanwhile, bring another pan of water to the boil and blanch the cabbage for a few minutes; drain and set aside. In the cabbage pan, heat the milk with the spring onions. Mash the potatoes with the butter, then mix this with the warm milk, spring onions and blanched cabbage; season well.
When the fish is ready, using a slice, transfer it to a large bowl and discard the bay leaves. Tip the fish cooking liquid into a jug. When the fish is cool, remove the skin and discard any bones. Break the fish into chunks and mix with the prawns and tomatoes.
Colcannon is an Irish take on mash – try it with sausages or gammon, too. If you want to lower the calories in this recipe, leave out the cheddar.
For the sauce, melt the butter in a pan, add the flour and stir over the heat for 1 minute. Take the pan off the heat and whisk in the fish cooking liquid, a little at a time. Return to the heat and simmer for 5 minutes until thickened. Take off the heat and stir in three quarters of the cheese. Mix with the fish, prawns and tomatoes. Transfer to a large buttered ovenproof baking dish.
Top with the colcannon and scatter with the remaining cheese. Bake for 45-55 minutes or until bubbling and lightly golden.