Make the potato topping. Cook the potatoes in a saucepan of salted boiling water for 15 minutes or until tender. Drain well and return to the pan. Mash together with the butter and cream and season well.
Heat the oil in a large saucepan, add the onion and fry for 2–3 minutes until beginning to soften. Add the spinach to the pan and cook until wilted and any liquid has evaporated.
Melt the butter in a saucepan, add the flour and cook, stirring, for 1 minute. Gradually add the milk and cook, stirring all the time, until thickened and smooth. Stir in the mustard and nutmeg and season well.
Arrange the fish and the prawns in a large ovenproof dish. Pour the sauce over the fish, spoon the mash on top and place in a preheated oven, 200°C (400°F), Gas Mark 6, for 30–35 minutes until golden and bubbling.