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Heat oven to 200C/180C fan/gas 6. Tip the spinach into a colander and sit it in the sink. Slowly pour a kettle of boiling water over the spinach to wilt it, then run under colder water, squeeze out as much water as you can and set aside.
Quarter the celeriac and cut each piece into thin slices. Butter a gratin dish and tip in the celeriac, spinach and the cream. Season, add the nutmeg and toss everything together. Cover the dish with foil and bake for 30 mins. Remove the foil, toss the celeriac around, cover again and put back in the oven for 30 mins.
Remove the foil again, lay the fish on top and place a small knob of butter on each fillet. Cover with foil again and bake for a final 20 mins until the fish is cooked and the celeriac is tender. Scatter everything with chives and serve.