Preheat the oven to 200°C/fan 180°C/gas mark 6. Put the cauliflower florets in a large roasting tin and drizzle with the oil. Roast in the oven for 30 minutes, until just tender. Remove from the oven and set aside.
Melt the butter in a saucepan until foaming, then add the flour. Reduce the heat to its lowest level and stir continuously for a couple of minutes. Combine the stock and milk, then add to the pan very slowly, whisking as you go to ensure the sauce is smooth. When all the liquid has been added, bring to the boil, then reduce to a simmer. Simmer for 10 minutes, stirring frequently, until the sauce has thickened. Add most of the parmesan, reserving some to scatter on top, the nutmeg and some freshly ground pepper.
Cook the frozen peas in boiling water for 3-4 minutes. Drain and roughly crush with a potato masher. Transfer the peas to the base of your pie dish. Mix the fish with the white sauce and pour into the pie dish on top of the peas. Cover the top with the roasted cauliflower and scatter over the reserved parmesan. Bake in the oven for 20 minutes, or until golden and bubbling.