Place the potatoes in a saucepan and cover with water. Add a large pinch of salt and bring to the boil. Gently simmer, partially covered with a lid, for about 20 minutes, or until the potatoes are completely tender. Drain thoroughly and return the potatoes to the pan over a low heat for a couple of minutes to steam off any remaining water. Use a potato ricer or masher to mash the potatoes. Beat in the egg yolks, 25g (1oz) butter and 50ml (2fl oz) milk and season to taste with freshly ground black pepper and salt.
Preheat oven to gas 4, 180°C, fan 160°C. Place the remaining butter in a small saucepan along with the milk, the flour and fish stock and gently bring to the boil, whisking constantly. Keep whisking as the mixture boils and thickens to form a smooth sauce. Simmer for 3 minutes. Remove from the heat and stir in the crème fraîche, tarragon and lemon zest and season with plenty of pepper and just a little salt.
Have a quick check for bones and then distribute the fish mix over the base of a medium, ovenproof dish. Pour the sauce evenly over the fish and spread the mash over the top, working from the outside in. Use a fork to rake over the top, creating a rough surface. Bake for 35 minutes, or until golden on top and bubbling beneath.