Melt the butter in a deep saucepan over medium-low heat. Add the flour and stir constantly for 6 minutes to cook out the raw flour taste and form a golden roux.
Bring the milk to a boil in a separate pot, then slowly pour it into the roux mixture while whisking vigorously to avoid lumps.
Bring the mixture to a boil, then lower the heat and cook for 10 minutes, stirring constantly. Remove from heat, stir in the salt and nutmeg, and set aside.
Bring a large pot of salted water to a boil. Add the potato slices and boil for about 7 minutes until they are slightly softened but still hold their shape. Drain and set aside.
Preheat your oven to 400°F (200°C). In a large bowl, combine the ground beef, finely chopped white onion, fresh parsley, and paprika. Season with salt and pepper, and mix well. Shape into round meatballs.
Arrange a single layer of potato slices at the bottom of a round baking dish. Place the meatballs over the potatoes, and use the remaining potato slices to separate the meatballs vertically.
Pour the warm béchamel sauce into each compartment until full, then top the entire dish with the shredded mozzarella.
Bake for 15 minutes until the cheese is melted, bubbling, and golden brown. Garnish with fresh parsley and serve.