- Preheat the oven to 2000C / 4000F, 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.