Preheat the oven to 200°C.
Boil the potatoes in salted water until tender, then drain and mash with butter, milk, and pepper.
Simmer the sliced leeks in melted butter for 10 minutes until soft.
Whisk the flour and wine together in a bowl until smooth.
Add milk to the leeks, bring to a boil, and stir in the wine mixture until thickened.
Season the sauce, add parsley and fish, then stir over heat for two minutes.
Pour mixture into a casserole dish, top with eggs, and let cool until firm.
Spread mashed potatoes over the fish, mark with a fork, and sprinkle with cheese.
Bake for 30–40 minutes until golden-brown and bubbling.