First, make the mash. Bring a pan of water to the boil and add salt. Tip in the potatoes, bring back to the boil, and simmer until tender (about 20-25 minutes), adding the celeriac after the first 5 minutes. When they’re cooked, strain in a colander, tip back into the pan and let them steam for 5 minutes to lose some of the moisture. Mash well, then beat in the butter. Season with salt and pepper and a few gratings of nutmeg.
While the potatoes are cooking, prepare the filling. Warm the butter in a large, heavy-based frying pan over a low-medium heat. Add the onions and leeks and sauté, stirring from time to time, until they’re softened (about 15 minutes). Sprinkle over the flour and stir for a couple of minutes, then pour in the wine or vermouth. Let it simmer for a couple of minutes then slowly stir in the milk, bring to a simmer and let it thicken for 5 minutes, stirring.
Remove from the heat, then fold in the fish, peas, spring onions and herbs. Season.
Heat the oven to 180°C fan/gas 6. Spoon the fish into an ovenproof dish. Spread the mash over the top, roughing it up slightly with a fork. Sprinkle on the cheese and breadcrumbs and bake for 25-30 minutes until golden and bubbling. Allow it to sit for about 10 minutes before serving as it’s volcanically hot