Place the potato into a pot of lightly salted water and simmer until soft. Drain water, then add milk and mash until smooth. Season with salt and pepper. Cover and set aside.
Mix bread crumbs with lemon juice and zest and pat firmly onto the fish.
Place in a hot oven and roast for 8 – 10 mins until the topping is crispy.
While the fish is cooking, gently fry leeks with the remaining butter over a medium heat until soft.
Remove fish from oven.
Mix chopped herbs through the mashed potato, and place a spoonful onto each plate. Add the fish on top of the mash and serve surrounded by the creamed leeks.