Before you start, preheat oven to 220°C (or 200°C fan bake). Bring a large pot of salted water to a rolling boil. Set aside a large baking dish.
Prep kumara. Add to pot of boiling water and cook on high heat for 15-20 minutes, until very tender. Drain well and return to pot to with milk. Mash until smooth and season with salt and pepper.
Prep cauliflower, leek and parsley. Pat fish dry and remove any scales or bones. Slice 3cm and season with salt and pepper. Lay fish in a single layer in baking dish.
Heat first measure of butter in medium pot with lid on medium-high heat. Cook leek with salt for 2-3 minutes, until starting to soften. Add cornflour and lemon pepper and cook for a further 30 seconds or until fragrant. Add cauliflower and stock, mix well and bring to boil. Cook with the lid on for about 5 minutes, until cauliflower is tender. Remove from heat, stir through peas, sour cream and parsley and season to taste with salt and pepper. Pour over fish.
Melt second measure of butter. In a medium bowl, combine breadcrumbs, Parmesan, salt and butter. Incorporate with your fingers to mix through butter. Top filling with kumara and scatter over topping.
Cook pie for 10-12 minutes, until top is crispy and fish is just cooked. Allow to cool, before cutting into portions.