In a large pot, bring the cream and stock to a boil, add salt, nutmeg and some white pepper. Add the potatoes and simmer, covered, until they are almost soft, stirring occasionally. Add the spinach and cook for a few more minutes.

Meanwhile, preheat the oven to 220 degrees Celsius. Sprinkle some salt on the skinless side of the fish filets. Heat some oil in a frying pan and fry the filets on the skin side until they are crunchy. Transfer them to a baking dish with the skinless sides on top, crumble butter on top, put them to the oven and bake until they are done (approximately 3-5 minutes). To serve, squeeze some lemon juice on top and offer with the potato and spinach casserole.