Heat the oven to 180C, (160F fan) 350F, Gas 4. Butter an ovenproof pie dish.
Lay the haddock and onion on the base and cover with the potato.
Make the sauce by melting the butter over a medium heat, reduce the heat and stir in the flour to make a smooth paste. Gradually stir in the milk and keep stirring till the sauce comes to the boil and thickens. Season to taste with salt and pepper. Turn off the heat and pour over the potatoes.
Mix the oats and melted butter together and season lightly. Sprinkle over the sauce.
Put the dish on a baking tray and bake for 25 to 30 minutes till bubbling and crisp on top.
Serve with a green vegetable.