1. Preheat the oven to 350F.
2. To make the mashed potatoes, peel the potatoes and cut each one into quarters. Place in a large pot and cover with cold water by 1 inch. Add the salt, then cover and bring to a boil. Reduce the heat and simmer, partially covered, for 15 minutes or until very tender. Drain, then mash until smooth. Stir in the milk and butter. Reserve.
3. To make the fish filling, melt the butter in a PADERNO Canadian Signature Stainless Steel Frying Pan Non-Stick, Oven Safe, 32cm over medium heat. Add the leeks and cook for 5 to 8 minutes or until very soft. Sprinkle in the flour and stir well to combine. Whisk in about half the milk, stirring until the sauce is smooth and thick. Whisk in the rest of the milk and cook gently, stirring occasionally, until thickened, about 3 minutes. Whisk in the cream and salt. Stir in the peas, parsley and lemon zest. Stir in the fish.
4. Remove from heat and dollop the mashed potatoes overtop, smoothing them into an even layer. Sprinkle potatoes with cheese. Transfer the pan to the oven and bake for 30 minutes or until bubbly at the edges.