Fish pie with sliced potato topping

Method

1. Cook the potato slices in a large pan of salted boiling water for 7-8 minutes until just tender, then drain.
2. Heat ½ tbsp oil in a medium pan over a medium-high heat and fry the onion for 6-7 minutes until softening, then add the garlic and fry for a minute more. Remove the onion to a plate and set aside.
3. Heat the oven to 200°C/ 180°C fan/gas 6. Melt the butter in the same pan, then add the flour and beat in with a wooden spoon. Cook for 2-3 minutes, stirring, until it starts to smell biscuity. Slowly add the milk, whisking constantly, then whisk gently for 3-4 minutes until thickened. Remove the pan from the heat and stir in the parsley, onion mixture and a good pinch of black pepper.
4. Gently stir in the fish, then transfer to a 1.3 litre ovenproof dish. Lay the potato slices over the mixture, overlapping slightly, then brush with the remaining oil. Scatter over the cheddar and cook for 20 minutes or until lightly browned and bubbling.