Heat a frying pan over medium-high heat. Add the minced pork and cook for 5 minutes, breaking it apart. Add the finely diced onion and cook for 2-3 minutes more until the meat is browned with no pink remaining (63°C / 145°F) and the onion is softened. Season well with salt, pepper, and paprika powder.
Transfer the pork and onion to a baking dish. In the same frying pan, fry the sauerkraut over medium heat for 1-2 minutes. Spread the sauerkraut evenly over the pork and onion in the baking dish.
Cook the peeled potatoes in salted water for 15-20 minutes until tender. Drain well and mash while still hot.
Stir the milk into the mashed potatoes and season with salt, pepper, paprika powder, and nutmeg. Add the egg and stir to combine thoroughly, then stir in the butter.
Spread the potato mixture evenly over the sauerkraut. Decorate the top with a fork.
Bake at 180°C (356°F) for 45 minutes.
Pour the cream evenly over the casserole and bake for 15 minutes more until the top is golden brown.
Remove from the oven and let cool for 2-3 minutes before serving.