Smoky fish and potato layer

Ingredients
455g (1lb) smoked cod, haddock or whiting fillets, skinned and cubed
30g (1oz) butter or margarine
1 onion, finely chopped
1 clove garlic, crushed
30g (1oz) flour
300ml (10fl oz) milk
55g (2oz) grated cheese
2 large courgettes, sliced
680g (1lb 8oz) sweet potatoes or home-grown potatoes, peeled, sliced and par-boiled
Serves 4 

Method
Preheat the oven to 200°C/400°F, Gas Mark 6
Melt the butter or margarine in a saucepan.  Add the onion and garlic and cook until soft.  Stir in the flour and cook for 1-2 minutes, remove from the heat and gradually add the milk.
Return to the heat and cook, stirring until the sauce is thickened.
Remove from the heat, stir in half the cheese and all the fish.
Arrange half the courgettes and potatoes in a layer onto the base of a shallow ovenproof dish.  Spoon over the fish mixture.
Top with the remaining courgettes and potatoes, sprinkle with the cheese.
Bake for 25-30 minutes and serve with seasonal vegetables.