1. Heat the oven to 200C.
2. Heat the oil in a pan. Add the leeks and soften for 4-5 mins. Add the flour and stir in for 2 mins.
3. Remove the pan from the heat and gradually stir in the milk. Return to the heat and cook, stirring continuously until thickened. Simmer for 5 mins.
4. Stir in the mustard and seasoning. Pour in a pie dish, stirring in the fish pieces.
5. Gently fry the potato slices in a little olive oil. Season and top the fish pie with the slices.
6. Place the pie on a baking sheet and cook for 35-40 mins until the potatoes are golden and the fish cooked through.