Preheat oven to 200 c (400 f Gas 6)
Boil the potatoes in a pan of salted water until soft. Drain well and mash. Add butter and mash again.
Put the fish in a shallow frying pan. Season well with salt and black pepper. Pour just enough milk to cover and poach over a medium heat for 3-4 min. Remove the fish with a slotted spoon and transfer to an ovenproof dish.
For the Sauce,
Melt the butter in a pan over a low heat. Remove from the heat and stir in the flour with a wooden spoon until smooth. Return to the heat and add the milk little by little, stirring constantly. Keep cooking and stirring for 5-10 min until sauce has thickened.
Add more milk if needed.
Stir thought the mustard and season with salt and pepper.
Add the peas and eggs (if using) and stir gently
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Spoon the sauce on top of the fish and stir gently
Top with the mash potato and fork the top so that the potato forms peaks