Preheat the oven to Gas Mark 3, 170°C, fan 150°C. Grease a shallow ovenproof dish.
Wash the spinach, put it in a large saucepan, cover and cook on a medium heat until the leaves are just wilted, shaking the pan from time to time. Drain well, allow to cool and squeeze out excess water. Roughly chop and put in a bowl. Add the nutmeg and paprika, season and mix well.
Slice the potatoes as thinly as possible, ideally using a mandolin, and put half in the greased dish. Season.
Spread the spinach over the potatoes, sprinkle over half the cheese, then layer the remaining potatoes on top. Mix the milk and crème fraîche, then pour over the potatoes and spinach.
Sprinkle evenly with the remaining cheese and bake for about 1¼ hours until golden brown and cooked through.