Preheat the oven to 200°C /fan 180°C /Gas 6. Place the potato cubes in a medium saucepan and cover with water. Bring to the boil and simmer for 6 minutes until the potatoes are tender but still holding their shape. Drain well and pat dry with kitchen towel.
Meanwhile, heat the butter in a medium saucepan and when melted add the leeks, cook gently for 5 minutes until starting to soften and then add the flour and cook for a further minute. Slowly pour in the milk and stirring constantly until you have a smooth thickened sauce.
Remove the sauce from the heat and stir in the herbs and grated lemon zest and the cubed fish and mix well. Spoon into a 1½ litre ovenproof dish.
Place the potato cubes over the top of the fish and then scatter over the breadcrumbs, cheese and drizzle over the extra virgin olive oil. Bake for 30 minutes until the potatoes are golden and crunchy. Serve with spring greens and peas.
Serve with shredded spring greens and peas.