Smoked fish pie

4 large potatoes, peeled and cut into chunks
50g Butter, plus extra for buttering dish
1 small Onion, finely sliced
3 Tbsp Flour
2 cups Milk, full cream
310g tinned Smoked fish

Preheat the oven to 190C lightly butter an ovenproof baking dish.
Cook the potatoes in a saucepan of boiling water until tender. Drain and mash with ΒΌ cup milk and 1 Tbsp butter. Set aside.
Melt the butter in a heavy-based saucepan and add the onion. Cook gently until the onion is soft, about 5 minutes. Remove from the heat, add the flour and stir well. Slowly work in the milk.
Place back over a medium heat, stirring continuously and cook until the sauce is thick. Season with salt and pepper. Add the fish.
Spoon mixture into a shallow 8-cup ovenproof dish. Spread with mashed potato, fluffing surface with a fork for a crust. Dot potato with remaining spread and bake for 30 minutes, until golden. Serve.