Preheat the oven 200C/400F/Gas 6. Grease a shallow 2.5 litre/4Â½pint dish with butter.
Melt the butter in a pan on a high heat and fry onion for few minutes. Lower the heat, cover and leave to soften for 15 minutes.
Remove the lid and increase the heat to remove any moisture. Sprinkle in the flour and then add the milk gradually, stirring well and allowing the sauce to thicken until all the milk has been added.
Season with salt and freshly ground black pepper, add lemon juice and the raw fish and cook for couple of minutes, stirring continuously until the fish has just cooked. Stir in the smoked haddock. Pour the mixture into the buttered dish and leave to cool. Top with the hard-boiled eggs.
For the topping, boil the potatoes in a pan of salted water until tender. Drain, tip the potatoes back into the pan, add milk and butter and mash until lump-free. Use a ricer to get smooth mash. Add salt and freshly ground black pepper to taste.
Spread the mash over the top of the sauce and fish in the dish and score with a fork.
Bake in the preheated oven for about 30 minutes, or until the potato is crisp and golden-brown.